WBCHSE Class 12 Physics Electromagnetic Waves Multiple Choice Questions

Class 12 Physics Electromagnetic Waves Multiple Choice Questions And Answers

Question 1. One requires 11ev of energy to dissociate a carbon monoxide molecule into carbon and oxygen atoms. The minimum frequency of the appropriate electromagnetic radiation to achieve dissociation lies in

  1. Visible region
  2. Infrared region
  3. Ultraviolet region
  4. Microwave region

Answer: 3. Ultraviolet region

E = hf and c = fλ

∴ \(\lambda=\frac{h c}{E}\)

=\(\frac{6.63 \times 10^{-34} \times 3 \times 10^8}{11 \times 1.6 \times 10^{-19}}\) = 1.13 x 10-7 m

‍∴ The wave lies in the ultraviolet region.

Question 2. A linearly polarized electromagnetic wave given by \(\vec{E}=E_0 \hat{i} \cos (k z-\omega t)\) is incident normally on a perfectly reflecting infinite wall at z = a. Assuming that the material of the wall is optically inactive, the reflected wave will be given as

  1. \(\vec{E}_r=-E_0 \hat{i} \cos (k z-\omega t)\)
  2. \(\vec{E}_r=E_0 \hat{i} \cos (k z+\omega t)\)
  3. \(\vec{E}_r=-E_0 \hat{i} \cos (k z+\omega t)\)
  4. \(\vec{E}_r=E_0 \hat{i} \sin (k z-\omega t)\)

Answer: 2. \(\vec{E}_r=E_0 \hat{i} \cos (k z+\omega t)\)

The phase difference between the incident and reflected wave being n, the reflected ray will be \(\vec{E}_r=E_0 \hat{i} \cos (k z+\omega t)\).

Question 3. An EM wave radiates outwards from a dipole antenna, with E0 as the amplitude of its electric field vector. The electric field E0 which transports significant energy from the source falls off as

  1. \(\frac{1}{r^3}\)
  2. \(\frac{1}{r^2}\)
  3. \(\frac{1}{r}\)
  4. Remains constant

Answer: 3. \(\frac{1}{r}\)

Question 4. The ratio of contributions made by the electric field and magnetic field components to the intensity of an EM wave is

  1. c:1
  2. c2:1
  3. 1:1
  4. √c:1

Answer: 3. 1:1

Question 5. A plane electromagnetic wave propagating along x -direction can have the following pairs of \(\vec{E}\) and \(\vec{B}\)

  1. Ex, By
  2. Ey, Bz
  3. Bx, By
  4. Bz, By

Answer:

2. Ey, Bz,

3. Ez, Ey

Since the wave is traveling along the x-direction, \(\vec{E}\) and \(\vec{B}\) will be along y and z -direction respectively, and vice versa.

Question 6. A charged particle oscillates about its mean equilibrium position with a frequency of 109 Hz. The electromagnetic wave produced

  1. Will have a frequency of 109 Hz
  2. Will have a frequency of 2 x 109 Hz
  3. Will have a wavelength of 0.3 m
  4. Fall in the region of radio waves

Answer:

  1. Will have a frequency of 109 Hz
  2. Will have a frequency of 2 x 109 Hz
  3. Will have a wavelength of 0.3 m

The frequency of electromagnetic waves is equal to the frequency of oscillating charged particles.

Question 7. The source of electromagnetic waves can be a charge

  1. Moving with a constant velocity
  2. Moving in a circular orbit
  3. At rest
  4. Falling in an electric field

Answer:

2. Moving in a circular orbit

3. At rest

Accelerated charged particles can produce electromagnetic waves.

Question 8. μ0 and ε0 are the magnetic permeability and the electric permittivity, respectively, of free space. Φ is the electric flux across any Gaussian surface. Then the displacement current is defined as

  1. \(\frac{d \phi}{d t}\)
  2. \(\epsilon_0 \frac{d \phi}{d t}\)
  3. \(\mu_0 \frac{d \phi}{d t}\)
  4. \(\mu_0 \epsilon_0 \frac{d \phi}{d t}\)

Answer: \(\epsilon_0 \frac{d \phi}{d t}\)

Question 9. Electric flux enclosed by a surface A is given by

  1. \(\epsilon_0 \int \vec{E} \cdot d \vec{A}\)
  2. \(\frac{1}{\epsilon_0} \int \vec{E} \cdot d \vec{A}\)
  3. \(\epsilon_0 \mu_0 \int \vec{E} \cdot d \vec{A}\)
  4. \(\int \vec{E} \cdot d \vec{A}\)

Answer: 2. \(\frac{1}{\epsilon_0} \int \vec{E} \cdot d \vec{A}\)

Question 10. Electromagnetic waves are produced by

  1. A static charge
  2. A uniformly moving charge
  3. An accelerated charge
  4. Neutral particles

Answer: 3. An accelerated charge

Question 11. Of the following frequencies, which one may be the frequency of a radio wave?

  1. 102HZ
  2. 108HZ
  3. 1014HZ
  4. 1020HZ

Answer: 2. 108HZ

Question 12. Of the following frequencies, which one may be the frequency of X-rays?

  1. 102HZ
  2. 108HZ
  3. 1014HZ
  4. 1020HZ

Answer: 4. 1020HZ

Question 13. Of the following frequencies, which one may be the frequency of an infrared wave?

  1. 102HZ
  2. 108HZ
  3. 1014HZ
  4. 1020HZ

Answer: 3. 1014HZ

Question 14. Wavelengths of microwave, ultraviolet, and infrared rays are λm, λn, and λi respectively. Which one of the following is correct?

  1. λmni
  2. λinm
  3. λnim
  4. λmin

Answer: 4. λmin

Question 15. Which of the following is not an electromagnetic wave?

  1. Cosmic ray
  2. γ-ray
  3. β-ray
  4. x-ray

Answer: 3. β-ray

Question 16. Which of the following has the shortest wavelength?

  1. Microwaves
  2. Ultraviolet rays
  3. X-rays
  4. Infrared rays

Answer: 3. X-rays

Question 17. The frequency orders of y-rays, X-rays, and UV rays are a, b, and c respectively. Which of the following is correct?

  1. a>b, b<c
  2. a>b, b>c
  3. a<b, b>c
  4. a = b = c

Answer: 2. a>b, b>c

Question 18. The decreasing order of the wavelength of infrared, microwave, ultraviolet, and gamma rays is

  1. Microwave, infrared, ultraviolet, gamma rays
  2. Gamma rays, ultraviolet, infrared, microwaves
  3. Microwaves, gamma rays, infrared, ultraviolet
  4. Infrared, microwave, ultraviolet, gamma rays

Answer: 1. Microwave, infrared, ultraviolet, and gamma rays

Production And Propagation Of Electromagnetic Waves

Question 19. Electromagnetic wave is a kind of

  1. Matter-wave
  2. Stationary wave
  3. Longitudinal wave
  4. Progressive wave

Answer: 4. Progressive wave

Question 20. Which phenomenon proves that electromagnetic waves are transverse waves?

  1. Polarisation
  2. Interference
  3. Reflection
  4. Diffraction

Answer: 1. Polarisation

Question 21. The ratio between the amplitudes of electric and magnetic fields at any point on a progressive electromagnetic wave in free space is equal to

  1. \(\frac{1}{\mu_0 \epsilon_0}\)
  2. \(\frac{1}{\sqrt{\mu_0 \epsilon_0}}\)
  3. \(\sqrt{\mu_0 \epsilon_0}\)
  4. \(\mu_0 \epsilon_0\)

Answer: 2. \(\frac{1}{\sqrt{\mu_0 \epsilon_0}}\)

Question 22. In a plane electromagnetic wave, the electric field (E) having an amplitude of 48 V.m-1 oscillates at a frequency of 2.0 x 1010 Hz. The amplitude of the oscillating magnetic field (B) is

  1. 3.2 x 10-8 T
  2. 3 x 107 T
  3. 16 x 10-7 T
  4. 1.6 x 10-7 T

Answer: 4. 1.6 x 10-7 T

Question 23. The electric and the magnetic field associated with an em wave propagating along the +z -axis can be represented by

  1. \(\left[\vec{E}=E_0 \hat{i}, \vec{B}=B_0 \hat{j}\right]\)
  2. \(\left[\vec{E}=E_0 \hat{k}, \vec{B}=B_0 \hat{i}\right]\)
  3. \(\left[\vec{E}=E_0 \hat{j}, \overrightarrow{B_1}=B_0 \hat{i}\right]\)
  4. \(\left[\vec{E}=E_0 \hat{j}, \vec{B}=B_0 \hat{k}\right]\)

Answer: 1. \(\left[\vec{E}=E_0 \hat{i}, \vec{B}=B_0 \hat{j}\right]\)

Question 24. An electromagnetic wave in a vacuum has the electric and magnetic fields \(\vec{E}\) and \(\vec{B}\), which are always perpendicular to each other. The direction of polarization is given by \(\vec{X}\) and that of wave propagation by \(\vec{k}\). Then

  1. \(\vec{X} \| \vec{E} \text { and } \vec{k} \| \vec{E} \times \vec{B}\)
  2. \(\vec{X} \| \vec{B} \text { and } \vec{k} \| \vec{E} \times \vec{B}\)
  3. \(\vec{X} \| \vec{E} \text { and } \vec{k} \| \vec{B} \times \vec{E}\)
  4. \(\vec{X} \| \vec{B} \text { and } \vec{k} \| \vec{B} \times \vec{E}\)

Answer: 1. \(\vec{X} \| \vec{E} \text { and } \vec{k} \| \vec{E} \times \vec{B}\)

Question 25. The electric field associated with an em wave in vacuum is given by \(\vec{E}=\hat{i} 40 \cos \left(k z-6 \times 10^8 t\right)\), where E, z and t are in V.m-1, meter and seconds respectively. The value of wave vector k is

  1. 2 m-1
  2. 0.5 m-1
  3. 6 m-1
  4. 3 m-1

Answer: 1. 2 m-1

Question 26. Electromagnetic waves

  1. Can show interference
  2. Can be polarised
  3. Are deflected by an electric field
  4. Are deflected by the magnetic field

Answer:

1. Can show interference

2. Can be polarised

Question 27. During the propagation of electromagnetic waves in a vacuum, the electric field \(\vec{E}\) and the magnetic field \(\vec{B}\) at each point

  1. Are mutually perpendicular
  2. Are you in the same phase
  3. Varry an equal amount of energy by dividing the average energy of the wave between them
  4. The ratio of amplitude of these fields is equal to the speed of light

Answer: All are correct

Question 28. When the electromagnetic wave enters into a medium from free space,

  1. The velocity of the wave decreases
  2. The frequency of the wave decreases
  3. The wavelength of the wave decreases
  4. Frequency increase and wavelength decreases

Answer:

2. Frequency of the wave decreases

3. The wavelength of the wave decreases

Question 30. c and v are the velocities of an electromagnetic wave in free space of permittivity e0 and permeability and a medium of permittivity e and permeability fi respectively. If the refractive index of the medium is n, then which of the following relations are correct?

  1. \(c=\frac{1}{\sqrt{\mu_0 \epsilon_0}}\)
  2. \(v=\frac{1}{\sqrt{\mu \epsilon}}\)
  3. \(n=\frac{v}{c}\)
  4. \(n=\sqrt{\frac{\mu \epsilon}{\mu_0 \epsilon_0}}\)

Answer:

1. \(c=\frac{1}{\sqrt{\mu_0 \epsilon_0}}\)

2. \(v=\frac{1}{\sqrt{\mu \epsilon}}\)

4. \(n=\sqrt{\frac{\mu \epsilon}{\mu_0 \epsilon_0}}\)

Question 31. On the surface of the earth, the average intensity of sunlight is 1300 W. m-2. If the electric permittivity of free space or air is 8.845 x 10-12 F.m-1,

  1. The average amplitude of the electric field on the earth’s surface is almost 990 V.m-1
  2. The average amplitude of the magnetic field on the earth’s surface is almost 3.3 x 10-8 Wb.m-2
  3. The average energy density of sunlight on the earth’s surface is almost 4.33 x 10-6 J.m-3
  4. In case of normal incidence, the polarisation surface is parallel to the earth’s surface

Answer:

1. The average amplitude of the electric field on the earth’s surface is almost 990 V.m-1

3. The average energy density of sunlight on the earth’s surface is almost 4.33 x 10-6 J.m-3

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