Sastry CBSE

Current Electricity Important Questions for CBSE Class 12 Physics Potentiometer, Cell and their Combinations

1.Cell A device to maintain a steady current in an electric circuit is electrolytic cell. It has two electrodes positive (F) and negative (N) as shown in figure below.

2.Internal Resistance The electrolyte through which a current flows has a finite resistance r, called the internal resistance.

3.The emf of cell is the potential difference between two terminals of cell when there is no flow of current through it.
The potential difference between two terminals of cell when current flows through it, is known as terminal voltage or terminal potential difference (V) of cell.

13.The sensitivity of a potentiometer refers to the capability of measuring very small potential difference and exhibit change in balancing length even on very small change in potential difference.
The sensitivity of potentiometer can be increased by increasing the number of wires of potentiometer and hence, decreasing the value of potential gradient.

14.The potentiometer works only when(i) the terminal voltage applied by driving cell is greater than the emf of primary cell. (ii) the positive terminals of driving cell and primary cell are connected at the zero end of potentiometer wire.

15.The potentiometer is a better device to measure potential difference than a voltmeter as null point method is used and hence, it can measure even the emf of cell but voltmeter cannot. It measures potential difference with greater accuracy.

Previous Year Examination Questions

1 Mark Questions

1.State the underlying principle of a potentiometer?  [Delhi 2014 c]
Ans.The potentiometer works on the principle that potential difference across any two points of uniform current carrying conductor is directly proportional to the length between the two points.

2.Two identical cells, each of emf E, having negligible internal resistance, are connected in parallel with each other across an external resistance What is the current through this resistance? [All India 2013]
Ans.The cells are arranged as shown in the circuit

As the internal resistance of cells is negligible, so total resistance of the circuit = R
So. current through the resistance. I=E/R
(In parallel combination, potential is same as the single cell)

Ans.Since, the positive terminal of the batteries are connected together, so the equivalent emf of the batteries is given by £ = 200 -10 = 190 V
Hence, the current in the circuit is given by  I=E/R=190/38=5 A

4.The emf of a cell is always greater than its terminal voltage. Why? Give reason.          [Delhi 2013]
Ans.The emf of a cell is greater than its terminal  voltage because there is some potential drop across the cell due to its small internal resistance

5.A cell of emf E and internal resistance r draws a current Write the relation between terminal voltage V in terms of E, I and r.[Delhi 2013]
Ans.When a current Idraws from a cell of emf E and internal resistance r, then the terminal voltage is
V = E – Ir.

6.A resistance R is connected across a cell of emf E and internal resistance r. Now, a potentiometer measures the potential difference between the terminals of the cells as V. Write the expression for r in terms of E, V and R.

Ans.

7.A (i) series (ii) parallel combination of two given resistors is connected, one-by-one, across a cell. In which case, will the terminal potential difference across the cell have a higher value?[All India 2008 C]
Ans.The equivalent resistance combination of resistances is (i) greater than the greatest resistance in series combination and (ii) smaller than the least value of resistance in parallel combination.
The terminal potential difference across the cell is higher in series combination as V = E – Ir and due to higher resistance, current I is less in series combination.

8.The plot of the variation of potential difference across a combination of three identical cells in series versus current is as shown in figure. What is the emf of each cell?

Ans.Terminal potential difference across a cell can be obtained by subtracting potential drop across internal resistance of the cell from the emf of the cell.
v Terminal voltage across cell combination,
V = E – Ir
when current I=0
=>     V = E
From graph, when I = 0, V = 6 V

=>    emf E = 6 V

2 Marks Questi0ns

9.A cell of emf E and internal resistance r is connected across a variable resistor Plot a graph showing variation of terminal voltage V of the cell versus the current I. Using the plot, show the emf of the cell and its internal resistance can be determined.[All India 2014]
Ans.

Ans.

Ans.Refer to ans. 10. (Ans. 1.2V).

Ans.Refer to ans. 10. (Ans. 2.25 V).

Ans.

14.Describe briefly with the help of a circuit diagram, how a potentiometer is used to determine the internal resistance of a cell.[All India 2013]
Ans.

15.Two students X and Y perform an experiment on potentiometer separately using the circuit given below

Keeping other parameters unchanged, how will the position of the null point be affected if
(i)X increases the value of resistance R in the set up by keeping the key closed and the key K₂ open?
(ii)Y decreases the value of resistance S in the set up, while the key K₂ remains open and then K_t closed?
Justify your answer.[HOTS; Foreign 2012]
Ans.

16.Two cells of emf 2E and E and internal resistances 2r and r respectively, are connected in parallel. Obtain the expressions for the equivalent emf and the internal resistance of the combination.[All India 2010 C]
Ans.

17.Three cells of emf E,2E and 5 Eh aving internal resistances r, 2r and 3r, variable resistance R as shown in the figure. Find the expression for the current. Plot a graph for variation of current with R.
[All India 2010 C]

Ans.

18.A cell of emf E and internal resistance r is connected across a variable resistor R. Plot a graph showing the variation of terminal potential V with resistance!?.[Delhi 2009]
Ans.

19.Plot a graph showing the variation of terminal potential difference across a cell of emf E and internal resistance r with current drawn from it. Using this graph, how does one determine the emf of the cell?  [Delhi 2009 c]
Ans.

3 Marks Questions

Ans.

21.(i) State the underlying principle of a potentiometer. Why is it necessary to (i) use a long wire, (ii) have uniform area of cross-section of the wire and (iii) use a driving cell whose emf is taken to be greater than the emfs of the primary cells?
(ii) In a potentiometer experiment, if the area of the cross-section of the wire increases uniformly from one end to the other, draw a graph showing how potential gradient would vary as the length of the wire increases from one end.[All India 2014 C]
Ans.(i) Principle of Potentiometer The potential drop across the length of a steady current carrying wire of uniform cross-section is proportional to the length of the wire.
(a)We use a long wire to have a lower value of potential gradient (i.e a lower “least count” or greater sensitivity of the potentiometer.
(b)The area of cross-section has to be uniform to get a ‘uniform wire’ as per the principle of the potentiometer.
(c)The emf of the driving cell has to be greater than the emf of the primary cells as otherwise, no balance point would be obtained

22.In the figure, a long uniform potentiometer wire AB is having a constant potential gradient along its length. The null points for the two primary cells of emfs E_x and E₂ connected in the manner shown, are obtained at a distance of 120 cm and 300 cm from the end A

Find (i) E1 / E₂ and
(ii) position of null point for the cell E1
How is the sensitivity of a potentiometer increased? [Foreign 2014; Delhi 2012]
Ans.

23.State the underlying principle of a potentiometer. Write two factors on which the sensitivity of a potentiometer depends.

In the potentiometer circuit shown in the figure, the balance point is at X. State, giving reason, how the balance point is shifted when
(i)resistance R is increased
(ii)resistance S is increased, keeping R constant?[Compartment 2013]
Ans.We use a long wire to have a lower value of potential gradient (i.e a lower “least count” or greater sensitivity of the potentiometer  The two factors on which the sensitivity of a potentiometer depends are
(a)the value of potential gradient (K)
(b)by increasing the length of potentiometer wire. From the circuit diagram,
(i) if R is increased, the current through the potentiometer wire will decrease. Due to it, the potential gradient of potentiometer wire will also decrease. Thus, the position of J will shift towards B.
(ii) if S is increased, keeping R constant, the position of J will shift towards A.

Ans.

25.With the help of circuit diagram, explain how a potentiometer can be used to compare emf of two primary cells?  [Delhi 2011]
Ans.

26.State the underlying principle of a potentiometer. Describe briefly, giving the necessary circuit diagram, how a potentiometer is used to measure the internal resistance of a given cell?[Foreign 2011]
Ans.Principle of Potentiometer The potential difference across any two points of current carrying , wire, having uniform cross-sectional  area and material of the potentiometer is directly proportional to the length between the two points


The circuit diagram of potentiometer for determining internal resistance of a given cell is shown

Measurment of Internal Resistance of a Cell Using Potentiometer

27.Two cells of emf E_x, E₂ and internal resistances r_a and r₂ respectively are connected in parallel as shown in the figure.

Deduce the expressions for
(i)the equivalent emf of the combination.
(ii)the equivalent resistance of the combination and
(iii)the potential difference between the points A and [Foreign 2010]
Ans.

28.Draw the circuit diagram of a potentiometer which can be used to determine the internal resistance r of a given cell of emf Explain briefly how the internal resistance of the cell is determined? [Delhi 2010,2008 C]
Ans.

29.Write any two factors on which internal resistance of a cell depends. The reading on a high resistance voltmeter, when a cell is connected across it, is 2.2 V. When the terminals of the cell are also connected to a resistance of 5 ohms as shown in the circuit, the voltmeter reading drops to 1.8 V. Find the internal resistance of the cell.

Ans.The high resistance voltmeter means that no current with flow through it hence, there is no potential difference across it. So, the reading shown by the high resistance voltmeter can be taken as the emf of the cell.
The internal resistance of a cell depends on
(i) the concentration of electrolyte and
(ii) distance between the two electrodes

30.(i)State the principle of working of a potentiometer.
(ii)Figure shows the circuit diagram of a potentiometer for determining the emf of E cell of negligible internal resistance.

(a)What is the purpose of using high resistance ?
(b)How does the position of balance point (J) change when the resistance i?_x is decreased?
(c)Why cannot the balance point be obtained

• when the emf E is greater than 2 V.
• when the key (K) is closed?[Foreign 2009]

Ans.(i) Principle of Potentiometer The potential drop across the length of a steady current carrying wire of uniform cross-section is proportional to the length of the wire.
(a)We use a long wire to have a lower value of potential gradient (i.e a lower “least count” or greater sensitivity of the potentiometer.
(b)The area of cross-section has to be uniform to get a ‘uniform wire’ as per the principle of the potentiometer.
(c)The emf of the driving cell has to be greater than the emf of the primary cells as otherwise, no balance point would be obtained

(ii) (a) To protect the galvanometer from flow of high current.
(b) Balance point, J shift towards A
(c)Potential drop across the cell cannot become equal to potential difference across any two point of the curve. Potential difference across cell become zero

31.A circuit using a potentiometer and battery of negligible internal resistance is set up as shown to develop a constant potential gradient along the wire Two cells of emfs E₁ and E₂ are connected in series as shown in combinations (1) and (2). The balance points are obtained respectively at 400 cm and 240 cm from the point A. Find
(i)E₁ /E₂.
(ii)balancing length for the cell E1 only.

Ans.

32.A number of identical cells, n each of emf E,  internal  resistance r, connected in series are charged by a DC source of emf E’, using a resistor, R.
(i)Draw the circuit arrangement.
(ii)Deduce the expressions for
(a)the changing current and
(b)the potential difference across the combination of the cells. [Delhi 2008]
Ans.

33.A potentiometer wire of length lm is connected to a driver cell of emf 3 V as shown in the figure. When a cell of 1.5 V emf is used in the secondary circuit, the balance point is found to be 60 cm. On replacing this cell and using a cell of unknown emf, the balance point shifts to 80 cm.

(i)Calculate unknown emf of the cell.
(ii)Explain with reason, whether the circuit works, if the driver cell is replaced with a cell of emf 1 V.
(iii)Does the high resistance R, used in the secondary circuit affect the balance point? Justify your answer.[Delhi 2008]
Ans.

Ans. (i) Net emf applied in the circuit = Applied potential difference – Total emf of all cells = 100 V-4 x 8 V = 68 V
NOTE During changing of the current, positive terminal of the battery is connected to positive terminal of the series combination of the cells.

5 Marks Questions

35.(i) State the working principle of a potentiometer. With the help of the circuit diagram, explain how a potentiometer is used to compare the emf’s of two primary cells. Obtain the required expression used for comparing the emfs.
(ii) Write two possible causes for one sided deflection in a potentiometer experiment.[Delhi 2013]
Ans.(i) Working principle of potentiometer When a constant current is passed through a wire of uniform area of cross-section, the potential drop across any portion of the wire is directly proportional to the length of that portion.

(ii) (a) The emf of the cell connected in main circuit may not be more than the emf of the primary cells whose emfs are to be compared.
(b) The positive ends of all cells are not connected to the same end of the wire

36.State the working principle of a potentiometer. Draw a circuit diagram to compare emf of two primary cells. Derive the formula used.
(i)Which material is used for potentiometer wire and why?
(ii)How can the sensitivity of a potentiometer be increased? [Delhi 2011]
Ans.(i) For working principle of potentiometer
We use a long wire to have a lower value of potential gradient (i.e a lower “least count” or greater sensitivity of the potentiometer.
For circuit diagram to compare emf of two cells

(ii) Constantan or manganin (alloy) as they have low temperature coefficient of resistance.
(iii) The sensitivity of potentiometer can be increased by increasing the number of wires of potentiometer and hence, decreasing the value of potential gradient
Important Questions for Class 12 PhysicsClass 12 PhysicsNCERT Solutions Home Page

Ray Optics and Optical Instruments Important Questions for CBSE Class 12 Physics Optical Instrument

1.Near Point (D) The minimum distance at which an object can be seen most distinctly without any strain. For a healthy normal eye, it is 25 cm.

2.Simple Microscope (Magnifying glass) It is a converging lens of small focal length.

3.Compound Microscope It consists of two convex lenses, one objective of very small focal length with short aperture and one eyepiece, E of moderate focal length and large aperture

4.Reflecting Astronomical Telescope These telescopes form image free from chromatic aberration and spherical aberration.

5.Refracting Astronomical Telescope It consists of an objective lens of a large focal length (f₀) and large aperture, also an eye lens of small aperture and focal length.

Previous Years’ Examinations Questions

2 Marks Questions

1.Draw a ray diagram of a reflecting type telescope. State two advantages of this telescope over a refracting telescope. [ Delhi 2014 C; All India 2008C; Delhi 2008]

Ans.

Advantages

(i) Reflecting telescopes have high      resolving power due to a large aperture of mirrors.

(ii) Due to availability of paraboloidal mirror, the image is free from chromatic and     spherical aberration.

2.Draw a schematic diagram of refracting telescope. Write its two important limitations.  [Delhi 2014c]

Ans.

Limitations of refracting telescope over a reflecting type telescope.

(i) Refracting telescope suffers from chromatic aberration uses large sized lenses.

(ii) It is difficult and expensive to make such large sized lenses.

3.Draw a ray diagram for the formation of image by a compound microscope. Write the expression for total magnification when the image is formed at infinity.        [Delhi 2014c]

Ans. A compound microscope consists of two convex lenses coaxially separated by some distance. The lens nearer to the object is called the objective. The lens through which the final image is viewed is called the eyepiece. The focal length of objective lens is smaller than eyepiece.

4.A convex lens of focal length 25 cm is placed coaxially in contact with a concave lens of focal length 20 cm. Determine the power of the combination. Will the system be converging or diverging in nature? [Delhi 2013]

5.Draw a schematic arrangement of a reflecting telescope (Cassegrain) showing how rays coming from a distant object are received at the eyepiece. Write its two important advantages over a refracting telescope. [Delhi 2013C]

Ans. Diagram of a reflecting telescope (Cassegrain) is shown as below:

Advantages of reflecting telescope over a refracting telescope are given as below:

(i) A reflecting telescope reflects all wavelengths of light at the same angle, so there are no colour halos.

(ii) A mirror has only one surface to be figured, so it is easier to control the shape.

(iii) A mirror reflects the light, so the material that is made from does not have to be transparent ultraviolet light reflects equally well.

6.Two convex lenses of same focal length but of apertures A_x and A₂ (A₂ < A_r), are used as the objective lenses in two astronomical telescopes having identical eyepieces. What is the ratio of their resolving power? Which telescope will you prefer and why? Give reason. [Delhi 2011]

7.Define the resolving power of a telescope. Write any two advantages of a reflecting telescope over a refracting telescope.           [Delhi 2010 c]

8.Draw a labelled ray diagram of an astronomical telescope in the near point position. Write the expression for its magnifying power.      [All India 2008]

Ans. Ray diagram of an astronomical telescope in the near point position is

9.Draw a ray diagram of an astronomical telescope in the normal adjustment position. State two drawbacks of this type Of telescope. [Delhi      2008]

Drawbacks

(i) Image formed by these telescopes have chromatic and spherical aberration.

(ii) The length of telescope is very large in normal adjustment.

10.Draw a ray diagram of compound microscope. Write the expression for its magnifying power.    [Delhi      2008]
Ans.


11.(i) Draw a labelled ray diagram showing the formation of a final image by a compound microscope at least distance of distinct vision, (ii) The total magnification produced by a compound microscope is 20. The magnification produced by the eyepiece is 5. The microscope is focussed on a certain object. The distance between the objective and eyepiece is observed to be 14 cm. If least distance of distinct vision is 20 cm. Calculate the focal length of the objective and the eyepiece. [Delhi 2014 C]

12.Draw a labelled ray diagram of a refracting telescope. Define its magnifying power and write the expression for it. Write two important limitations of a refracting telescope over a reflecting type telescope.           [All India 2013]

Limitations of refracting telescope over a reflecting type telescope.

(i) Refracting telescope suffers from chromatic aberration uses large sized lenses.

(ii) It is difficult and expensive to make such large sized lenses.
13.Draw a ray diagram showing the image formation by a compound microscope. Hence, obtain expression for total magnification when the image is formed at the infinity. [Delhi 2013]

Ans. A compound microscope consists of two convex lenses coaxially separated by some distance. The lens nearer to the object is called the objective. The lens through which the final image is viewed is called the eyepiece. The focal length of objective lens is smaller than eyepiece.

14.A compound microscope uses an objective lens of focal length 4 cm and eyepiece lens of focal length 10 cm. An object is placed at 6 cm from the objective lens. Calculate the magnifying power of the compound microscope. Also, calculate the length of the microscope.     [All India 2011]

Ans.

NOTE

1. The separation between objective and eye lens is known as length of microscope.
2. The image formed by objective is an object for eye lens.

15.A giant reflecting telescope at an observatory has an objective lens of focal length 15 m. If an eyepiece lens of focal length 0 cm is used, find the angular magnification of the telescope. If this telescope is used to view the moon, what is the diameter of the image of the moon formed by the objective lens? The diameter of the moon is 3.42 x 10⁶ m and the radius of the lunar orbit is 3.8 X 10⁸ m.            [All India 2011]

Ans.

16.Two convex lenses of focal length 20 cm and 1 cm constitute a telescope. The telescope is focused on a point which is 1 m away from the objective. Calculate the magnification produced and the length of the tube if the final image is formed at a distance 25 cm from the eyepiece.         [Delhi 201 ic]

Ans.

17.The objective of an astronomical telescope has a diameter of 150 mm and a focal length of 4.00 m. The eyepiece has a focal length of 25.00 mm. Calculate the magnifying and resolving power of telescope. (X = 6000 A for yellow Colour).   [Delhi 2011C]

18.(i) Draw a    neat    labelled    ray diagram of an astronomical telescope in normal adjustment. Explain briefly its working.

(ii) An astronomical telescope uses two lenses of powers 10 D and ID. What is its magnifying power in normal adjustment? [All India 2010]

Ans.

Refer to ans. 2.

The image formed by objective lens must fall on the focus of eye lens in order to form final image at infinity.

19.i) Draw a    neat    labelled    ray diagram of a compound microscope. Explain briefly its working.

(ii) Why must both the objective and the eyepiece of a compound microscope have short focal lengths?    [All India 2010]

Ans.  (i) Refer to ans. 3.

The magnification by compound microscope is two step process.

Firstly, the objective gives a magnified image of the object and after that the eye piece produces the angular magnification.

20.Draw a schematic diagram of a reflecting telescope (Cassegrain). Write two important advantages that the reflecting telescope has over a refracting type.        [Foreign 2010]

Ans.

21.Explain with the help of a ray diagram, the working of an astronomical telescope. The magnifying power of a telescope in its normal adjustment is If the length of the telescope is 105 cm in this adjustment, find the focal lengths of the two lenses. [Ail India 2010C]

Ans.  For figure refer to ans. 2.

The parallel beam of light from distant object get focused in focal plane of objective at angle a. This image A’B’ acts as an object for eye lens which lie between optical centre of eye lens and its focus. Eyepiece form virtual, erect, magnified image A” B” at least distant of distinct vision.

5 Marks Questions

22.Draw a labelled ray diagram showing the image formation of a distant object by refracting telescope,

Deduce the expression for its magnifying power when the final image is formed at infinity.

(ii) The sum of focal lengths of the two lenses of a refracting telescope is 105 cm. The focal length of one lens is 20 times that of the other. Determine the total magnification of the telescope when the final image is formed at infinity. [All India 2014]

Ans.

23.Define magnifying power of a telescope. Write its expression. A small telescope has an objective lens of focal length 150 cm and an eyepiece of focal length 5 cm. If this telescope is used to view a 100 m high tower 3 km away, find the height of the final image when it is formed 25 cm away from the eyepiece.    [Delhi 2012]

Ans.The magnifying power of a telescope is equal to the ratio of the visual angle subtended at the eye by final image formed at least distance of distinct vision to the visual angle subtended at naked eye by the object at infinity.

24.How is the working of a telescope different from that of a microscope? The focal lengths of the objective and eyepiece of a microscope are 1.25 cm and 5 cm. respectively. Find the position of the object relative to the objective in order to obtain an angular magnification of 30 in normal adjustment.                       [Delhi 2012]



25. Draw a ray diagram to show the working of a compound microscope. Deduce an expression for the total magnification when the final image is formed at the near point.

In a compound microscope, an object is placed at a distance of 1.5 cm from the objective of focal length 1.25 cm. If the eyepiece has a focal length of 5 cm and the final image is formed at the near point. Estimate the magnifying power of the microscope. [Delhi 2010]

Ans.For figure Refer to ans. 3.

The objective lens forms real, inverted magnified image A’B’ of object AB in such a way that AB’ fall some where between pole and focus of eye lens. So, A’B’ acts as an object for eye lens and its virtual magnified image A” B” formed by the lens.

The magnifying power of a compound microscope is defined as the ratio of the visual angle subtended by final image at eye ((3) and the visual angle subtended by object at naked eye when both are at the least distance of distinct vision from the eye.

26.(i) (a) Draw a labelled ray diagram to show the formation of image in an astronomical telescope for a distant object.

(b) Write the three distinct advantages of a reflecting type telescope over a refracting type telescope.

(ii) A convex lens of focal length 10 cm is placed coaxially 5 cm away from a concave lens of focal length 10 cm. If an object is placed 30 cm in front of the convex lens. Find the position of the final image formed by the combined system. [All India 2009]

27.Draw the labelled ray diagram for the formation of image by an astronomical telescope.

Derive the expression for its magnifying power in normal adjustment. Write two basic features which can distinguish between a telescope and a compound microscope. [Foreign 2009]

28.(i) Draw a ray diagram for the formation of image by a compound microscope. Define

its magnifying power. Deduce the expression for the magnifying power of the microscope.

(ii) Explain

• Why must both the object and the eyepiece of a compound microscope have short focal lengths?
• While viewing through a compound microscope, why should our eyes be positioned not on the eyepiece but a short distance away from it for best viewing?      [Foreign 2008]

Ans. (i) For figure Refer to ans. 3.

For magnifying power refer to ans. 25

(ii) (a) Refer to ans. 19 (ii).

(b) When eyes are positioned at short distance away from eyepiece, then the image formed at infinity can be seen which is more suitable and comfortable for viewing by the relaxed eye
Important Questions for Class 12 PhysicsClass 12 PhysicsNCERT Solutions Home Page

Ray Optics and Optical Instruments Important Questions for CBSE Class 12 Physics Reflection, Refraction and Dispersion of Light

1.Ray Optics or Geometrical Optics In this optics, the light is considered as a ray which travels in a straight line. It states that for each and every object, there is an image.

2.Reflection Reflection is the phenomenon of changing the path of light without any change in the medium.

3.Reflection of Light The returning back of light in the same medium from which it has come after striking a surface is called reflection of light.

4.Laws of Reflection

Two laws of reflection are given as below:

(i) The angle of incidence i is equal to the angle of reflection r.

(ii) The incident ray, reflected ray and normal to the reflecting surface at the point of incidence all lie in the same plane.

5. Total number of images formed by two plane mirrors inclined at an angle 0 with each other is given by

6.Reflecting surface of a spherical mirror is a part of a hollow sphere. Spherical mirrors are of two types, (i) Concave spherical mirror (ii) Convex spherical mirror.

7.Sign Convention All measurements should be taken from pole of mirror. All measurements along the direction of incident ray will be positive and opposite to incident ray are negative. All the measurements for the distances above the principal axis are taken as positive and below the principal axis are taken as negative.

8.For a real object, u is negative whereas v is negative for real image and positive for virtual image.

9.Mirror Formula Mirror formula is a relation between focal length of the mirror and distances of objects and image from the mirror.

where, f = focal length, u = distance of the object from mirror, v = distance of the image from mirror.

10.Focal length of mirror (f) = Radius of curvature (R)/2

f=R/2

11.Linear Magnification The ratio of the size of the image formed by a spherical mirror I to the size of the object O is called the linear magnification produced by the spherical mirror.

where, I = height of image and O = height of object

12.Magnification (m) It is negative corresponding to real image and positive for virtual image.

13.Refraction The phenomenon of changing in the path of light as it goes from one medium to another is called refraction.

14.Laws of Refraction

Two laws of refraction are given as below:

• The incident ray, refracted ray and the normal to the refracting surface at the point of incidence lie in the same plane.
• The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for the two given media. This constant is denoted by n and is called the relative refractive index.

n =sin i/sin r

where, n is refractive index of the second medium when first medium is air.

15.Refractive index of medium 2 w.r.t. medium 1 is denoted as ₁n₂ and defined as

17.Total Internal Reflection (TIR) When a ray of light travelling from denser medium to rarer medium is incident at the interface of two medium at an angle greater than the critical angle for the two media, the ray is totally reflected back to denser medium. This phenomenon is called Total Internal Reflection. It occurs only when angle of incidence in denser medium is greater (not equal) than critical angle, i.e. i> i_c.

18.Principle of reversibility of light states that when final path of a ray of light after any number of reflections and refractions is reversed, the ray retraces its entire path.

22.Optical fibre, mirage, sparkling of diamond, totally reflecting prism, etc. work on the principle of total internal reflection.

24.Lens is a transparent medium bounded by two surfaces of which one or both surfaces are spherical.

(i) Convex or Converging Lens A lens which is thicker at the centre and thinner at its end is called convex lens.

Convex lenses are of three types which are given as below

(ii) Concave or Diverging Lens A lens which is thinner at the centre and thicker at its ends is called a concave lens.

Concave lenses are of three types which are given as below:

Previous Years Examination Questions

1.A biconvex lens made of a transparent material of refractive index 1.25 is immersed in a water of refractive index 1.33. Will the lens behave as a converging or a diverging lens? Give reason. [All India 2014]

Ans. When a lens is placed in a liquid, where refractive index is more than that of the material of lens, then the nature of the lens changes. So, when a biconvex lens of refractive index 1.25 is immersed in water (refractive index 1.33), i.e. in the liquid of higher refractive index, its nature will change. So, biconvex lens will act as biconcave lens or diverging lens.

2.A biconvex lens made of a transparent material of refractive index 1.5 is immersed in a water of refractive index 1.33. Will the lens behave as a converging or a diverging lens? Give reason. [Ail India 2014]

Ans. A biconvex lens acts as a converging lens in air because the refractive index of air is less than that of the material of the lens. The refractive index of water is less than the refractive index of the material of the lens (1.5). So, its nature will not change, it behaves as a converging lens.

3.A convex lens is placed in contact with a plane mirror. A point object at a distance of 20 cm on the axis of this combination has its image coinciding with itself. What is the focal length of the lens? [All India 2014]

Ans. The adjacent figure shows a convex lens L in contact with a plane mirror P is the point object kept in the front of this combination at a distance of 20 cm from it.

Since, the image is coinciding with the object itself, the rays from the object after refraction from the lens fall normally on the mirror M and form an image coinciding with the object itself. So, the image is formed at the focus of the lens. So, focal length of the lens is 20 cm.

4.Write the relationship between angle of incidence i, angle of prism A and angle of minimum deviations from a triangular prism. [Delhi 2013]

Ans.The relation between the angle of incidence /’, angle of prism, A and the angle of minimum deviation, A_m for a triangular prism is given by

5.How does focal length of a lens change when red light incident on it is replaced by violet light? Give reason for your answer.            [Foreign 2012]

Ans.The refractive index of the material of a lens increases with the decrease in wavelength of the incident light. So, focal length will decrease with decrease in wavelength according to the formula.

Thus, when we replace red light with violet light then due to increase in wavelength the focal length of the lens will decrease

6.Name the physical quantity which remains same for microwaves of wavelength 1 mm and UV-radiation of 1600 A in vacuum.     [Delhi 2012]

Ans. Both microwave and UV-rays are a part of the electromagnetic spectrum. Thus, the physical quantity that remains for both types of radiation will be their speeds equal to c. c=3×10⁸m/s

7.Under what condition, does a biconvex lens of glass having a certain refractive index act as a plane glass sheet when immersed in a liquid?   [Delhi 2012]

Ans. When refractive index of lens is equal to the refractive index of liquid.

8.For the same value of angle of incidence, the angles of refraction in three media A, B and C are 15°, 25° and 35° respectively. In which medium, would the velocity of light be minimum? [All India 2012]

9.When monochromatic light travels from one medium to another, its wavelength changes but frequency remains the same. Explain. [Delhi 2011]

Ans.Because refractive index for a given pair of media depends on the ratio of wavelengths and velocity of light in two medium and not on frequency

10.The refractive index of diamond is much greater than that of glass. How does a diamond cutter make use of this fact?         [HOTS; All India 2011C]

Ans. The refractive index of diamond is much higher than that of glass. Due to high refractive index, the critical angle for diamond air interface is low. The diamond is cut suitably so that the light entering the diamond from any face suffers multiple total internal reflections at the various surfaces.

11.If a ray of light propagates from a rarer to a denser medium, how does its frequency change? [All India 2011c]

Ans. Frequency remains unchanged when light travels from one transparent medium to another transparent medium.

12.State the criteria for the phenomenon of total internal reflection of light to take place. [Delhi 2011,2010, 2008C]

Ans. Following are the criteria for total internal reflection

(i) Light must pass from a denser to a rarer medium.

(ii) Angle of incidence must be greater than critical angle

13.A lens behaves as a converging lens in air and a diverging lens in water (g = 4/3). What will be the condition on the value of refractive index (g) of the material of the lens? [Delhi 2011c]

Ans. Refractive index of the material of lens is less than the refractive index of water.

14.A converging lens axially in contact with a diverging lens; both the lenses being of equal focal lengths. What is the focal length of the combination?      [All India 2010]

15.A glass lens of refractive index 1.45 disappears when immersed in a liquid  What is the . value of refractive index of the liquid? [Delhi 2010]

16.Calculate the speed of light in a medium whose critical angle is 30°. [Delhi 2010]

17.Why does the sky appear blue? [Foreign 2010]

18.Under what condition does the formation of rainbow occur? [All India 2010C]

Ans. Availability of rain drops causes refraction, dispersion and total internal reflection of sun light results in the form of rainbow and the back of the observer should be towards the sun.

19.Two thin lenses of power + 6 D and -2D are in contact. What is the focal length of the combination? [All India 2010]

20.Two thin lenses of power + 4 D and -2D are in contact. What is the focal length of the combination? [All India 2010]

21.Two thin lenses of power + 5D and -2.5D are in contact. What is the focal length of the combination? [All India 2010]

22.Why are convex mirrors used as side view mirrors in cars? [Delhi 2009c]

Ans. Because convex mirror forms virtual, erect and smaller image of object irrespective of relative position of object from mirror and therefore, its field of view is very wide.

23.A converging lens of refractive index 1.5 is kept in a liquid medium having same refractive index. What would be the focal length of the lens in this medium? [hots; Delhi 2008]

24.How does the power of convex lens vary if the incident red light is replaced by violet light? [Delhi 2008]

25.Explain giving reason why the sun looks reddish at sunrise or sunset? [Foreign 2008]

Important Questions for Class 12 PhysicsClass 12 PhysicsNCERT Solutions Home Page

Current Electricity Important Questions for CBSE Class 12 Physics Resistance and Ohm’s Law

1.The directed rate of flow of electric charge through any cross-section of a conductor is known as electric current.

where, n = number of charged particles constitute the current
NOTE Current is a scalar quantity
I is in the direction of flow of positive charge and opposite to the direction of flow of negative charge.
SI unit of current is ampere and is represented by A.
Ampere is defined as,

2.The current density at a point in a conductor is the ratio of the current at that point in the conductor to the area of cross-section of the conductor at that point provided the area is held normal to the direction of flow of current.

NOTE Current density is a vector quantity

3.Flow of Electric Charge in Metallic Conductors Among the solids, all metals are good conductors of electricity. The cause of conductance is free electrons.
In Case of a Solid Conductor (i.e. Cu, Fe, Ag, etc) atoms are tightly bound to each other. There are large number of free electrons in them.
In Case of a Liquid Conductor Like electrolytic solution, there are positive and negative charged ions which can move on applying electric field.

4.Drift Velocity It is defined as the average velocity with which the free electrons move towards the positive end of a conductor under the influence of an external electric field applied

Previous Year Examination Questions

1 Mark Questions

1.Plot a graph showing variation of current versus voltage for the material GaAs. [Delhi 2014]
Ans.

2.Show variation of resistivity of copper as a function of temperature in graph. [Delhi 2014; All India 2014]
Ans.Graph of resistivity of copper as a function of temperature is given below (resistivity of metals increases with increase in temperature)

3.Define the term drift velocity of charge carriers in a conductor and write its relationship with the current flowing through it. [Delhi 2014]
Ans.The term drift velocity of charge carriers in a conductor is defined as the average velocity acquired by the free electrons along the length of a metallic conductor under a potential difference applied across the conductor.Its relationship is expressed as

where, I is current flowing through the conductor,n is concentration of free electrons
e is electron i.e.charge
A is cross-sectional area

4.Define the term electrical conductivity of a metallic wire. Write its SI unit.  [Delhi 2014]
Ans.

5.Show variation of resistivity of Si with temperature in graph.[Delhi 2014]
Ans.

6.Define the term mobility of charge carriers in a conductor. Write its SI unit.  [Delhi 2014]
Ans.

7.Write a relation between current and drift velocity of electrons in a conductor. Use this relation to explain how the resistance of a conductor changes with the rise in temperature ? [Compartment  2013]
Ans.Relation between current and drift velocity of electrons in a conductor is given by l = Anev_d
where l = current, A = area of conductor, n = number density of electrons and v_d = drift velocity.
with the increase in temperature of a metallic conductor, resistance increases and hence, drift velocity decreases

8.When electrons drift in a metal from lower to higher potential, does it mean that all the free electrons of the metal are moving in the same direction?  [Delhi 2012]
Ans.No, the drift speed of electrons is superposed over the random velocities of the electrons

9.Show on a graph, the variation of resistivity with temperature for a typical semiconductor. [Delhi 2012]
Ans.

10.Two wires of equal length, one of copper and the other of manganin have the same resistance. Which wire is thicker? [All India 2012]
Ans.

11.Define resistivity of a conductor. Write its SI unit.  [All India 2011]
Ans.

Ans.

13.Two identical slabs, of a given metal, are joined together, in two different ways, as shown in figures
(a) and (b). What is the ratio of the resistances of these two combinations? [Delhi 2010 c]

Ans.

14.Two conducting wires X and Y of same diameter but different materials are joined in series across a battery. If the number density of electrons in X is twice than that in Y, then find the ratio of drift velocity of electrons in the two wires.[All India 2010]
Ans.

15.The three coloured bands, on a carbon resistor are red, green and yellow, respectively. Write the value of its resistance. [All India 2009c]
Ans.

16.Write an expression for the resistivity of a metallic conductor showing its variation over a limited range of temperatures. [Delhi 2008 C]
Ans.

17.Define ionic mobility. Write its SI Unit.[Foreign 2008]
Ans.

Ans.

2 Marks Questions

Ans.

Ans.

Ans.

22.Draw a plot showing the variation of resistivity of a (i) conductor and (ii) semiconductor, with the increase in temperature. How does one explain this behaviour in terms of number density of charge carriers and the relaxation time? [Delhi 2014 C]
Ans.

23.Derive an expression for the current density of a conductor in terms of the drift speed of electrons. [Foreign 2014]
Ans.Let potential difference V is applied across a conductor of length / and hence, an electric field E produced inside the conductor.

24.A conductor of length / is connected to a DC source of potential V. If the length of the conductor is tripled by gradually stretching it, keeping V constant, how will
(i)drift speed of electrons and
(ii)resistance of the conductor be affected? Justify your answer.[HOTS; Foreign 2012]
Ans.

25.Plot a graph showing temperature dependence of resistivity for a typical semiconductor. How is this behaviour explained? [Delhi 2011]
Ans.

Ans.

27.The sequence of coloured bands in two carbon resistors R_t and R₂ is
(i)brown, green, blue and
(ii)orange, black, green.
Find the ratio of their resistances.[Delhi 2010 C]
Ans.

Ans.

29.Derive an expression for drift velocity of free electrons in a conductor in terms of relaxation time. [Delhi 2009]
Ans.

30.Two metallic wires of the same material have the same length but cross-sectional area is in the ratio 1: 2. They are connected
(i)in series and
(ii)in parallel.
Compare the drift velocities of electrons in the two wires in both the cases.  [All India 2008]
Ans.

31.Derive an expression for the resistivity of a good conductor, in terms of the relaxation time of electrons.       [All India 2008]
Ans.

3 Marks Questions

Ans.(i)The term drift velocity of charge carriers in a conductor is defined as the average velocity acquired by the free electrons along the length of a metallic conductor under a potential difference applied across the conductor.Its relationship is expressed as

where, I is current flowing through the conductor,n is concentration of free electrons
e is electron i.e.charge
A is cross-sectional area

33.Define relaxation time of the free electrons drifting in a conductor. How it is related to the drift velocity of free electrons? Use this relation to deduce the expression for the electrical resistivity of the material.[All India 2012]
Ans.

Ans.

Ans.

Ans.

37.Three resistors R₁, R₂ and R₃ are connected in parallel, across a source of emf E and negligible internal resistance. Obtain a formula for the equivalent expressions for the current through each of the three resistors. [All India 2009 c]
Ans.

38.Prove that the current density of a metallic conductor is directly proportional to the drift speed of electrons.[Delhi 2008]
Ans.

Ans.

Important Questions for Class 12 PhysicsClass 12 PhysicsNCERT Solutions Home Page

Important Questions for CBSE Class 12 Physics Electromagnetic Waves

1.Displacement Current The current which comes into play m the region in which the electric field and the electric flux is changing with time. It is given by

2.Need for Displacement Current Ampere’s circuital law for conduction current during charging of a capacitor was found inconsistent. Therefore, Maxwell modified Ampere’s circuital law.

3.The displacement current produces in space due to change of electric flux linked with the surface. This reveals that, varying electric field is the source of magnetic field.

4.Maxwell’s Equations of Electromagnetic Waves Maxwell’s equations are the basic laws of electricity and magnetism. These equations give complete description of ail electromagnetic interactions.

5. Electromagnetic Waves An electromagnetic wave is a wave radiated by an accelerated or oscillatory charge in which varying magnetic field is the source of electric field and varying electric field is the source of magnetic field. Thus two fields becomes source of each other and the wave propagates in a direction perpendicular to both the fields.

6. Electromagnetic waves are transverse in nature, i.e. electric and magnetic fields are perpendicular to each other and to the direction of wave propagation. Electromagne­tic waves are not. deflected by electric and magnetic fields.

7. E (electric field) and B (magnetic field) in electromagnetic waves are in same phase.

8. Speed of electromagnetic wave

9. The energy in electromagnetic wave is divided on average equally between electric and magnetic fields.

10. Energy associated with an electromagnetic wave is

11. Linear momentum delivered to the surface p= U/c

where, U = total energy transmitted by electromagnetic waves and c = speed of electromagnetic wave

12. Electromagnetic Spectrum The systematic sequential distribution of electromagnetic waves in ascending or descending order of frequency or wavelength is known as electromagnetic spectrum. The range varies from 10^-12 m, to 10⁴ m, i.e. from y-rays to radio waves.

13. Elementary facts about the uses of electromagnetic waves

Radio waves

• In radio and TV communication.
• In astronomical field.

Microwaves

• In RADAR communication.
• In analysis of molecular and atomic structure.
•  For cooking purpose.

Infrared waves

• In knowing molecular structure.
•  In remote control of TV VCR, etc. Ultraviolet rays

Ultraviolet rays

• Used in burglar alarm.
• To kill germs in minerals.

X-rays

• In medical diagnosis as they pass through the muscles not through the bones.
• In detecting faults, cracks, etc., in metal products,

14. The optical effect is produced by electric field vector of the electromagnetic waves.

Previous Years’ Examination Questions

1 Mark Questions

1.To which part of the electromagnetic spectrum does a wave of frequency 5 x 10¹⁹ Hz belong?   [All India 2014]

Ans

2.To which part of the electromagnetic spectrum does a wave of frequency 3 x 10¹³ Hz belong?  [All India 2014]
Ans

3.Write the condition under which an electron will move undeflected in the presence of crossed electric and magnetic fields.   [All India 2014c]
Ans

4.Welders wear special goggles or face masks with glass windows to protect their eyes from electromagnetic radiation. Name the radiations and write the range of their frequency. [All India 2013]
Ans

5.What are the directions of electric and magnetic field vectors relative to each other and relative to the direction of propagation of electromagnetic waves? [All India 2012]

Ans

6.Name the electromagnetic waves which
(i) maintain the earth’s warmth and
(ii) are used in aircraft navigation. [Foreign 2012]
Ans

7.A plane electromagnetic wave travels in vacuum along Z- direction. What can you say about the direction of electric and magnetic field vectors? [Delhi 2011]
Ans

8.Write two uses of microwaves. [Foreign 2011]
Ans

9. Write two uses of infrared rays. [Foreign 2011]
Ans

10.Write two uses of X-rays. [Foreign 2011]
Ans

11.What is the frequency of electromagnetic waves produced by oscillating charge of frequency V=10⁵Hz?        [Delhi 2011C]
Ans

12.How are infrared waves produced? What is the range of their wavelength? [Delhi 2010c]
Ans

13.Which of the following has the shortest wavelength? Microwaves, ultraviolet rays, X-rays.   [All India 2010]
Ans

14. Arrange the following in descending order of wavelength. X-rays, radiowaves, blue light, infrared light.  [All India 2010]
Ans

15. Which part of electromagnetic spectrum has largest penetrating power?          [Delhi 2010]
Ans

16. Which part of electromagnetic spectrum is absorbed from sunlight by ozone layer? [Delhi 2010]
Ans

17. Which part of electromagnetic spectrum is used in RADAR systems?    [Delhi 2010]
Ans Micro Waves

18.Name the electromagnetic radiation used to destroy cancer cells and write its frequency range.  [Foreign 2010]
Ans

19.In what way, are the directions of the electric and magnetic field vectors representing an electromagnetic wave related to each other?           [Delhi 2010c]
Ans

20.Express the velocity of propagation of an electromagnetic waves in terms of the peak value of the electric and magnetic fields.    [All India 2010C]
Ans

21.Name the electromagnetic waves used for studying crystal structure of solids. What is its frequency range? [All India 2009]
Ans

22.Write the following radiations in ascending order in respect of their frequencies X-rays, microwaves, ultraviolet rays and radiowaves. [Delhi 2009]
Ans

23.Name the electromagnetic radiation to which waves of wavelength in the range of 10^-2 m belongs. Give one use of this part of electromagnetic spectrum. [Delhi 2009, 2008]
Ans

24.Name the electromagnetic radiation which can be produced by a klystron or a magnetron valve.    [Foreign    2009]
Ans

25.Why is ozone layer on top of the stratosphere crucial for human survival? [All India 2009C]
Ans

26.State the reason why microwaves are best suited for long distance transmission of signals.    [Foreign 2008]
Ans

27.Special devices like the klystron valve or the magnetron valve, are used for production of electromagnetic waves. Name these waves and also write one of their applications.                               [Delhi   2008C]

2 Marks Questions

28.(a) How does oscillating charge produce electromagnetic waves?
(b) Sketch a schematic diagram depicting oscillating electric and magnetic fields of an EM wave propagating along positive Z-direction. [Foreign 2014]

Ans

29.(i) How are electromagnetic waves produced?
(ii) How do you convince yourself that electromagnetic waves carry energy and momentum? [Delhi 2013c]
Ans

30.(i) Arrange the following electromagnetic waves in the descending order of their wavelengths.

• Microwaves
• Infrared rays
• Ultraviolet radiation
• y-rays

(ii) Write one use each of any two of them.    [Delhi 2013C]
Ans

31.(i) An electromagnetic wave is travelling in a medium, with a velocity v = vi. Draw a sketch showing the propagation of the electromagnetic wave, indicating the direction of the oscillating electric and magnetic fields.
(ii) How are the magnitudes of the electric and magnetic fields related to velocity of the electromagnetic wave? [HOTS; Delhi 2013; All India 2008C]
Ans

32.A capacitor of capacitance C is being charged by connecting it across a DC source along with an ammeter. Will the ammeter show a momentary deflection during the process of charging? If so, how would you explain this momentary deflection and the resulting continuty of current in the circuit? Write the expression for the current inside the capacitor.    [All India 2012]
Ans

33.When an ideal capacitor is charged by a DC battery, no current flows. However, when an AC source is used, the current flows continuously. How does one explain this, based on the concept of displacement current? [Delhi 2012]
Ans

34.Explain briefly how electromagnetic waves are produced by an oscillating charge? How is the frequency of the electromagnetic waves produced related to that of the oscillating charge? [Foreign 2012]
Ans

35.Name the constituent radiation of electromagnetic spectrum which is used for

• aircraft navigation
• studying the crystal structure Write the frequency range for each. [Delhi 2011C]

Ans

36.Draw a sketch of a plane electromagnetic wave propagating along the Z-direction. Depict clearly the directions of electric and magnetic fields varying sinusoidally with Z. [All India 2011]

37.Arrange the following electromag­netic radiations in ascending order of their frequencies.

• Microwaves
• Radio waves
• X-rays
• y-rays

Write two uses of any one of these. [Delhi 2010]
Ans

38.How are X-rays produced? Write their two important uses. [Foreign 2010;Delhi 2010]
Ans

39.How are infrared rays produced? Write their two important uses. [Foreign 2010; All India 2010 C]
Ans

40.How are microwaves produced? Write their two important uses. [Foreign 2010]

Ans

41.Answer the following questions.

• Optical and radio telescopes are built on the ground while X-ray astronomy is possible only from satellites orbiting the earth. Why?
• The small ozone layer on top of the stratosphere is crucial for human survival. Why?      [All India 2009]

Ans

42.How does a charge, q oscillating at certain frequency produce electromagnetic waves? Sketch a schematic diagram depicting electric and magnetic fields for an electromagnetic wave propagating along the Z- direction. [Delhi 2009]
Ans

43.Name the electromagnetic radiations having the wavelength range from 1 mm to 700 nm. Give its two important applications. [Foreign 2009]
Ans

44.Name the electromagnetic radiation having the wavelength range from 10^-1 m to 10~³ Give its two important applications. [Foreign 2009]
Ans

45.Name the electromagnetic radiations having the wavelength range from 1 nm to 10~³ Give its two important applications. [Foreign 2009]
Ans

46.Identify the different types of electromagnetic radiations, which are used

• to kill germs,
• for physical therapy. [All India 2009c]

Ans

47.In an electromagnetic wave, the oscillating electric field having a frequency of 3 x 10¹⁰ Hz and an amplitude of 30 V/m propagates in the positive X-direction.

• What is the wavelength of the electromagnetic wave?
• Write down the expression to represent the corresponding magnetic field.           [Foreign 2008]

Ans

48.The following table gives the wavelength range of some constituents of the electromagnetic spectrum.

S. No.	Wavelength range
1.	1 nm to 700 nm
2.	400 nm to 1 nm
3.	1 nm to10-3 nm
4.	<10“3 nm

Select the wavelength range and name of the electromagnetic waves that are

• widely used in the remote switches and household electronic devices.
• produced in nuclear reactions. [Delhi 2008C]

Ans

3 Marks Questions

49.Answer the following questions.

• Name the waves which are produced during radioactive decay of a nucleus. Write their frequency range.
• Welders wear special glass goggles while working. Why? Explain.
• Why are infrared waves often called as heat waves? Give their one application.  [Delhi 2014]

Ans

50.Answer the following questions
(i) Name the EM waves which s suitable for radar systems used in aircraft navigation. Write the range of frequency of these waves.
(ii) If the earth did not have atmosphere, would its average surface temperature be higher or lower than what it is now? Explain.
(iii) An EM wave exerts pressure on the surface on which it is incident. Justify.    [Delhi 2014]
Ans

51.Answer the following questions.

• Name the EM waves which are used for the treatment of certain forms of cancer. Write their frequency range.
• Thin ozone layer on top of stratosphere is crucial for human survival. Why?
• Why is the amount of the momentum transferred by the EM waves incident on the surface so small? [Delhi 2014]

Ans

52.Answer the following questions:

• Show, by giving a simple example, how EM waves carry energy and momentum.
• How are microwaves produced? Why is it necessary in microwaves ovens to select the frequency of microwaves to match the resonant frequency of water molecules?
• Write two important uses of infrared waves.        [Delhi 2014 c]

Ans

53.(i) Describe briefly how electromagnetic waves are produced by oscillating charges? (ii) Give one use of each of the following.

• Microwaves
• Ultraviolet rays
• Infrared rays
• y-rays [HOTS; All India 2011c]

Ans

54.How are electromagnetic waves produced by oscillating charges? Why is it not possible to produce electromagnetic waves in the visible region with modern electronic circuits in the laboratory? What is the method of production of X-rays?  [Delhi 2009C]
Ans

55.Identify the  following electromagnetic radiations as per the wavelengths given below. Write one application of each.

• 10^-3 nm
• 10^-3 m
• 1 nm [All India 2008]

Ans

56.Identify the   following electromagnetic radiations as per the frequencies given below. Write one application of each.

• 10²⁰ Hz
• 10¹² Hz
• 10ⁿ Hz [All India 2008]

Ans

Important Questions for Class 12 PhysicsClass 12 PhysicsNCERT Solutions Home Page