## 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**

- Visible region
- Infrared region
- Ultraviolet region
- 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**

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

**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 E _{0} as the amplitude of its electric field vector. The electric field E_{0} which transports significant energy from the source falls off as**

- \(\frac{1}{r^3}\)
- \(\frac{1}{r^2}\)
- \(\frac{1}{r}\)
- 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**

- c:1
- c
^{2}:1 - 1:1
- √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 \(\vec{B}\)**

- E
_{x}, B_{y} - E
_{y}, B_{z} - B
_{x}, B_{y} - B
_{z}, B_{y}

**Answer:**

2. E_{y}, B_{z},

3. E_{z}, E_{y}

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 10 ^{9} Hz. The electromagnetic wave produced**

- Will have a frequency of 10
^{9}Hz - Will have a frequency of 2 x 10
^{9}Hz - Will have a wavelength of 0.3 m
- Fall in the region of radio waves

**Answer:**

- Will have a frequency of 10
^{9}Hz - Will have a frequency of 2 x 10
^{9}Hz - 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**

- Moving with a constant velocity
- Moving in a circular orbit
- At rest
- Falling in an electric field

**Answer:**

2. Moving in a circular orbit

3. At rest

Accelerated charged particles can produce electromagnetic waves.

**Question 8. μ _{0} and ε_{0} 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**

- \(\frac{d \phi}{d t}\)
- \(\epsilon_0 \frac{d \phi}{d t}\)
- \(\mu_0 \frac{d \phi}{d t}\)
- \(\mu_0 \epsilon_0 \frac{d \phi}{d t}\)

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

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

- \(\epsilon_0 \int \vec{E} \cdot d \vec{A}\)
- \(\frac{1}{\epsilon_0} \int \vec{E} \cdot d \vec{A}\)
- \(\epsilon_0 \mu_0 \int \vec{E} \cdot d \vec{A}\)
- \(\int \vec{E} \cdot d \vec{A}\)

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

**Question 10. Electromagnetic waves are produced by**

- A static charge
- A uniformly moving charge
- An accelerated charge
- Neutral particles

**Answer:** 3. An accelerated charge

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

- 10
^{2}HZ - 10
^{8}HZ - 10
^{14}HZ - 10
^{20}HZ

**Answer:** 2. 10^{8}HZ

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

- 10
^{2}HZ - 10
^{8}HZ - 10
^{14}HZ - 10
^{20}HZ

**Answer:** 4. 10^{20}HZ

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

- 10
^{2}HZ - 10
^{8}HZ - 10
^{14}HZ - 10
^{20}HZ

**Answer:** 3. 10^{14}HZ

**Question 14. Wavelengths of microwave, ultraviolet, and infrared rays are λ _{m}, λ_{n}, and λ_{i} respectively. Which one of the following is correct?**

- λ
_{m}>λ_{n}>λ_{i} - λ
_{i}>λ_{n}>λ_{m} - λ
_{n}>λ_{i}>λ_{m} - λ
_{m}>λ_{i}>λ_{n}

**Answer:** 4. λ_{m}>λ_{i}>λ_{n}

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

- Cosmic ray
- γ-ray
- β-ray
- x-ray

**Answer:** 3. β-ray

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

- Microwaves
- Ultraviolet rays
- X-rays
- 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?**

- a>b, b<c
- a>b, b>c
- a<b, b>c
- 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**

- Microwave, infrared, ultraviolet, gamma rays
- Gamma rays, ultraviolet, infrared, microwaves
- Microwaves, gamma rays, infrared, ultraviolet
- 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**

- Matter-wave
- Stationary wave
- Longitudinal wave
- Progressive wave

**Answer:** 4. Progressive wave

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

- Polarisation
- Interference
- Reflection
- 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**

- \(\frac{1}{\mu_0 \epsilon_0}\)
- \(\frac{1}{\sqrt{\mu_0 \epsilon_0}}\)
- \(\sqrt{\mu_0 \epsilon_0}\)
- \(\mu_0 \epsilon_0\)

**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 10^{10} Hz. The amplitude of the oscillating magnetic field (B) is**

- 3.2 x 10
^{-8}T - 3 x 10
^{7}T - 16 x 10
^{-7}T - 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**

- \(\left[\vec{E}=E_0 \hat{i}, \vec{B}=B_0 \hat{j}\right]\)
- \(\left[\vec{E}=E_0 \hat{k}, \vec{B}=B_0 \hat{i}\right]\)
- \(\left[\vec{E}=E_0 \hat{j}, \overrightarrow{B_1}=B_0 \hat{i}\right]\)
- \(\left[\vec{E}=E_0 \hat{j}, \vec{B}=B_0 \hat{k}\right]\)

**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**

- \(\vec{X} \| \vec{E} \text { and } \vec{k} \| \vec{E} \times \vec{B}\)
- \(\vec{X} \| \vec{B} \text { and } \vec{k} \| \vec{E} \times \vec{B}\)
- \(\vec{X} \| \vec{E} \text { and } \vec{k} \| \vec{B} \times \vec{E}\)
- \(\vec{X} \| \vec{B} \text { and } \vec{k} \| \vec{B} \times \vec{E}\)

**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**

- 2 m
^{-1} - 0.5 m
^{-1} - 6 m
^{-1} - 3 m
^{-1}

**Answer:** 1. 2 m^{-1}

**Question 26. Electromagnetic waves**

- Can show interference
- Can be polarised
- Are deflected by an electric field
- 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**

- Are mutually perpendicular
- Are you in the same phase
- Varry an equal amount of energy by dividing the average energy of the wave between them
- 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,**

- The velocity of the wave decreases
- The frequency of the wave decreases
- The wavelength of the wave decreases
- 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?**

- \(c=\frac{1}{\sqrt{\mu_0 \epsilon_0}}\)
- \(v=\frac{1}{\sqrt{\mu \epsilon}}\)
- \(n=\frac{v}{c}\)
- \(n=\sqrt{\frac{\mu \epsilon}{\mu_0 \epsilon_0}}\)

**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},**

- The average amplitude of the electric field on the earth’s surface is almost 990 V.m
^{-1} - The average amplitude of the magnetic field on the earth’s surface is almost 3.3 x 10-8 Wb.m
^{-2} - The average energy density of sunlight on the earth’s surface is almost 4.33 x 10-6 J.m
^{-3} - 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}