CBSE Class 12 Physics Chapter-8 Important Questions – Free PDF Download
Free PDF download of Important Questions with Answers for CBSE Class 12 Physics Chapter 8 – Electromagnetic Waves 3 Marks Questions prepared by expert Physics teachers from latest edition of CBSE(NCERT) books only by CoolGyan to score more marks in CBSE board examination.
CBSE Class 12 Physics Chapter-8 – Electromagnetic Waves
CBSE Class 12 Physics Important Questions Chapter 8 – Electromagnetic Waves
3 Marks Questions
1. In a plane electromagnetic wave, the electric field oscillates sinusoid ally with a frequency of 
and amplitude 48V/m.
(a) What is the wavelength of the em. wave?
(b) Calculate the amplitude of the oscillating magnetic field.
(c) Calculate average energy density of the electromagnetic field of the wave?
Ans. (a) V =
EO = 48 V/m
(b) EO = cBO
(c) Energy density
2. Find the wavelength of electromagnetic waves of frequency 
in free space. Give two applications of the type of wave.
Ans. V =
These are infra-red radiations
Applications
(1) It keeps the earth warm.
(2) Infra-red lamps are used to treat muscular strains.
3. A plane monochromatic wave lies in the visible region. It is represented by the sinusoidal variation with time by the following components of electric field
Ex = 0, Ey = 4 sin 
Ez = 0
Where 
And 
is the wavelength of light.
(a) What is the direction of propagation of the wave?
(b) What is its amplitude?
(c) Compute the component of magnetic field?
Ans. (a) The direction of propagation of wave is along + x – axis.
(b) Amplitude = 4 units
(c) Component of magnetic of field
4. Write the characteristics of em waves? Write the expression for velocity of electromagnetic waves in terms of permittivity and permeability of the medium?
Ans. Characteristics of em waves
(1) It travels in free space with speed of light 
(2) Electromagnetic waves are transverse in nature.
Velocity of em waves in vacuum C
= 
5. The electric field of a plane electromagnetic wave in vacuum is represented by.
Ex = 0, Ey = 0.5 cos 
and Ez = 0
(a) What is the direction of propagation of electromagnetic wave?
(b) Determine the wavelength of the wave?
(c) Compute the component of associated magnetic field?
Ans. (a) The equation Ey = 0.5 cos
Represents wave is propagating along + x – axis
(b) Comparing equation with the standard one
Ey = EO cos w 
(c) Associated magnetic field is 
to electric field and the direction of propagation. Since wave is propagating along x – axis, electric field is along, y – axis
Thus, magnetic field is along z – axis
6. Find the wavelength of electromagnetic waves of frequency 
in free space. Give its two applications.
Ans.
These are Gamma Rays.
Applications
(1) These rays are used to get information regarding atomic structure.
(2) They have very high penetrating power so they are used for detection purpose
7. (1) State the condition under which a microwave oven heats up food items containing water molecules most efficiently?
(2) Name the radiations which are next to these radiations in em. Spectrum having (a) Shorter wavelength (b) Longer wavelength
Ans. (1) Frequency of the microwaves must be equal to the resonant frequency of the water molecules present in the food item.
(2) (a) visible light
(b) Microwaves
8. The terminology of different parts of the electromagnetic spectrum is given in the text. Use the formula E = 
(for energy of a quantum of radiation: photon) and obtain the photon energy in units of eV for different parts of the electromagnetic spectrum. In what way are the different scales of photon energies that you obtain related to the sources of electromagnetic radiation?
Ans. Energy of a photon is given as:
Where,
h = Planck’s constant =
c = Speed of light = 
m/s
= Wavelength of radiation
The given table lists the photon energies for different parts of an electromagnetic spectrum for different .
| (m) | E (eV) |
| 103 |  |
| 1 |  |
 |  |
 |  |
 |  |
 |  |
 |  |
The photon energies for the different parts of the spectrum of a source indicate the spacing of the relevant energy levels of the source.
9. About 5% of the power of a 100 W light bulb is converted to visible radiation. What is the average intensity of visible radiation
(a) at a distance of 1 m from the bulb?
(b) at a distance of 10 m?
Assume that the radiation is emitted isotropic ally and neglect reflection.
Ans. Power rating of bulb, P = 100 W
It is given that about 5% of its power is converted into visible radiation.
Power of visible radiation,
Hence, the power of visible radiation is 5W.
(a) Distance of a point from the bulb, d = 1 m Hence, intensity of radiation at that point is given as:
(b) Distance of a point from the bulb, 
= 10 m Hence, intensity of radiation at that point is given as:
10. Use the formula 
T= 0.29 cm K to obtain the characteristic temperature ranges for different parts of the electromagnetic spectrum. What do the numbers that you obtain tell you?
Ans. A body at a particular temperature produces a continuous spectrum of wavelengths. In case of a black body, the wavelength corresponding to maximum intensity of radiation is given according to Planck’s law. It can be given by the relation,
Where,
= maximum wavelength
T = temperature
Thus, the temperature for different wavelengths can be obtained as:
For =
;
For =
;
For = 
cm;
and so on.
The numbers obtained tell us that temperature ranges are required for obtaining radiations in different parts of an electromagnetic spectrum. As the wavelength decreases, the corresponding temperature increases.
