# Physics MCQs for Class 12 Chapter 8 Electromagnetic Waves

MCQs on Class 12 Physics Chapter 8 Electromagnetic Waves

Q.1. In electromagnetic waves the phase difference between electric and magnetic field vectors are
(a) zero
(b) 𝜋4
(c) 𝜋2
(d) π

Answer Answer:
(a) Electric and magnetic field vectors always vary in same phase.

Q.2. Which of the following has minimum wavelength?
(a) Blue light
(b) γ-rays
(c) infrared rays
(d) microwave

Answer Answer:
(b) γ-rays have maximum frequency so minimum wavelength among electromagnetic waves.

Q.3. The correct option, if speeds of gamma rays, X-rays and microwave are Vg, Vx an Vm respectively will be.
(a) Vg > Vx > Vm
(b) Vg < Vx < Vm
(c) Vg > V> Vm
(d) Vg = Vx = Vm

Answer Answer:
(d) All electromagnetic waves travel with the speed of light in space.

Q.4. Which of the following has maximum penetrating power?
(a) Ultraviolet radiation
(b) Microwaves
(c) γ-rays
(d) Radio waves

Answer Answer:
(c) γ-rays have maximum frequency and energy of proton, therefore maximum penetrating power.

Q.5. Electromagnetic waves travelling in a medium having relative permeability μr = 1.3 and relative permittivity Er = 2.14. The speed of electromagnetic waves in medium must be
(a) 1.8 × 108 ms-1
(b) 1.8 × 104ms1
(c) 1.8 × 106ms1
(d) 1.8 × 10² ms-1

Answer Answer:
(a)

Q.6. The quantity √𝜇0𝜀0 represents
(a) speed of sound
(b) speed of light in vacuum
(c) speed of e.m. waves
(d) inverse of speed of light in vacuum

Answer Answer:
(d)

Q.7. In electromagnetic wave if ue and um are mean electric and magnetic energy densities respectively, then Answer Answer:
(a) Energy is equally distributed among electric field and magnetic field

Q.8. Which of the following is called heat radiation?
(a) X-rays
(b) γ-rays
(c) Infrared radiation
(d) Microwave

Answer Answer:
(c)

Q.9. From Maxwell’s hypothesis, a charging electric field gives rise to
(a) an electric field.
(b) an induced emf.
(c) a magnetic field.
(d) a magnetic dipole.

Answer Answer:
(c) A charging electric field gives rise to a magnetic field.

Q.10. The ultra high frequency band of radio waves in electromagnetic wave is used as in
(a) television waves
(b) cellular phone communication
(c) commercial FM radio
(d) both (a) and (c)

Answer Answer:
(b)

Q.11. Electromagnetic waves are transverse in nature is evident by
(a) polarisation.
(b) interference,
(c) reflection.
(d) diffraction.

Answer Answer:
(a) Only transverse waves can be polarised.

Q.12. Which of the following are not electromagnetic waves?
(a) Cosmic rays
(b) γ-rays
(c) β-rays
(d) X-rays

Answer Answer:
(c) β-rays consists of electrons which are not electromagnetic nature.

Q.13. 10 cm is a wavelength corresponding to the spectrum of
ia) infrared rays
(b) ultraviolet rays
(c) microwaves
(d) X-rays

Answer Answer:
(c) Microwaves have wavelength around 10 cm.

Q.14. If 𝐸⃗  and 𝐵⃗  represent electric and magnetic field vector of the electromagnetic waves then the direction of propagation of the em wave is that of Answer Answer:
(c) 𝐸⃗ ×𝐵⃗

Q.15. The structure of solids is investigated by using
(a) cosmic rays
(b) X-rays
(c) γ-rays
(d) infrared rays

Answer Answer:
(b) X-rays are used to investigate structure of solids.

Q.16.. The condition under which a microwave over heats up a food item containing water molecules most efficiently is
(a) The frequency of the microwaves must match the resonant frequency of the water molecules.
(b) The frequency of the microwaves has no relation with natural frequency of the water molecules.
(c) Microwaves are heat waves, so always produce heating.
(d) Infrared waves produce heating in a microwave oven.

Answer Answer:
(a) When frequency of microwave matches with frequency of water molecules i.e., resonant condition. Maximum energy is transferred to water molecules as their K.E. energy.

Q.17. Which radiations are used in treatment of muscle ache?
(a) Infrared
(b) Ultraviolet
(c) Microwave
(d) X-rays

Answer Answer:
(a) Infrared radiations are used in the treatment of muscle ache.

Q.18. Waves in decreasing order of their wavelength are
(a) X-rays, infrared rays, visible rays, radio waves
(b) radio waves, visible rays, infrared rays, X-rays.
(c) radio waves, infrared rays, visible rays, X-rays.
(d) radio waves, ultraviolet rays, visible rays, X-rays.

Answer Answer:
(c)

Q>19. The electric field associated with an e.m wave in vacuum is given by 𝐸⃗  =40 cos (kz – 6 × 108𝑖⃗ , where E, Z and t are in volt/m, metre and seconds respectively. The value of wave vector K is
(a) 2 m-1
(b) 0.5 m-1
(c) 6 m-1
(d) 3 m-1

Answer Answer:
(a) Wave vector, 𝑘=𝑊𝐶=6×108/3×108=2 m−1

Q.20. Electromagnetic waves with wavelength λ are used by a FM radio station for broadcasting. Here λ belongs to
(a) radio waves
(b) VHF radio waves
(c) UHF radio waves
(d) microwaves

Answer Answer:
(b)

Q.21. The oscillating magnetic field in a plane electromagnetic wave is given as By = (8 × 10-6) sin [2 × 1011t+ 300πx] T, wavelength of the em wave is
(a) 0.80 cm
(b) 1 × 103m
(c) 2 × 10-2cm
(d) 0.67 cm

Answer Answer:
(d) wavelength 𝜆=2𝜋300𝜋=1150m = 0.67 cm

Q.22. Maxwell in his famous equations of electromagnetism introduced the concept of
(a) ac current
(b) displacement current
(c) impedance
(d) reactance

Answer Answer:
(b)

Q.23. The conduction current is same as displacement current when source is
(a) ac only
(b) dc only
(c) either ac or dc
(d) neither dc nor ac

Answer Answer:
(c)

Q.24. If a variable frequency ac source is connected to a capacitor then with decrease in frequency the displacement current will
(a) increase
(b) decrease
(c) remains constant
(d) first decrease then increase

Answer Answer:
(b)

Q.25. An electromagnetic wave can be produced, when charge is
(a) moving with a constant velocity
(b) moving in a circular orbit
(c) falling in an electric field
(d) both (b) and (c)

Answer Answer:
(d)

Q.26. Which of the following statement is false for the properties of electromagnetic waves?
(a) Both electric and magnetic field vectors attain the maxima and minima at the same place and same time.
(b) The energy in electromagnetic waves is divided equally between electric and magnetic field vectors.
(c) Both electric and magnetic field vectors are parallel to each other and perpendicular to the direction of propagation of wave.
(d) These waves do not require any material medium for propagation.

Answer Answer:
(c)

Q.27. Which of the following has/have zero average value in a plane electromagnetic wave?
(a) Both magnetic and electric fields
(b) Electric field only
(c) Magnetic field only
(d) None of these

Answer Answer:
(a)

Q.28. A charged particle oscillates about its mean equilibrium position with a frequency of 109 Hz. The frequency of electromagnetic waves produced by the oscillator is
(a) 106 Hz
(b) 107 Hz
(c) 108 Hz
(d) 109 Hz

Answer Answer:
(d)

Q.29. If E and B denote electric and magnetic fields respectively, which of the following is dimensionless? Answer Answer:
(a)

Q.30. The waves used by artificial satellites for communication is
(a) microwaves
(b) infrared waves
(c) radio waves
(d) X-rays

Answer Answer:
(a)

Q.31. Which of the following electromagnetic waves is used in medicine to destroy cancer cells?
(a) IR-rays
(b) Visible rays
(c) Gamma rays
(d) Ultraviolet rays

Answer Answer:
(c)

Q.32. A linearly polarized electromagnetic wave given as E = E0 𝑖̂  cos (kz – ωt) 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 [NCERT Exemplar]
(a) Er = -E0 𝑖̂  cos (kz – ωt).
(b) Er = E0 𝑖̂  cos (kz + ωt).
(c) Er = -E0 𝑖̂  cos (kz + ωt).
(d) Er = E0 𝑖̂  sin (kz – ωt).

Answer Answer:
(b)

Q.33. Light with an energy flux of 20 W/cm2 falls on a non-reflecting surface at normal incidence. If the surface has an area of 30 cm2, the total momentum delivered (for complete absorption) during 30 minutes is , [NCERT Exemplar]
(a) 36 × 10-5 kg m/s.
(b) 36 × 10-4 kg m/s.
(c) 108 × 104 kgm/s.
(d) 1.08 × 107 kg m/s.

Answer Answer:
(b)

Q.34. The electric field intensity produced by the radiations coming from 100 W bulb at a 3 m distance is E. The electric field intensity produced by the radiations coming from 50 W bulb at the same distance is [NCERT Exemplar] Answer Answer:
(d)

Q.35. If E and B represent electric and magnetic field vectors of the electromagnetic wave, the direction of propagation of electromagnetic wave is along [NCERT Exemplar]
(a) E.
(b) B.
(c) B × E.
(d) E × B.

Answer Answer:
(d)

Q.36. Which of the following statement is false for the properties of em waves?
(a) The energy of em wave is divided equally between electric and magnetic fields.
(b) Both electric and magnetic field vectors are parallel to each and perpendicular to the direction of propagation of wave.
(c) These waves do not require any material medium for propagation.
(d) Both electric and magnetic field vectors attain the maximum and minimum at the same place and same time.

Answer Answer:
(b) Electric and magnetic field vectors are mutually perpendicular to each other directed along the propagation of wave.

Q.37. 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 [NCERT Exemplar] Answer Answer:
(c)

Q.38. An electromagnetic wave travelling along z-axis is given as-E=E0 cos (kz – cot). Choose the incorrect option from the following;
(a) The associated magnetic field is given as B = 1𝑐𝑘̂  × E = 1𝜔(𝑘̂  × E)
(b) The electromagnetic field can be written in terms of the associated magnetic field as E = c(B × 𝑘̂ )
(c) 𝑘̂ .E = 0, 𝑘̂ .B =0.
(d) 𝑘̂  × E = 0, k𝑘̂  × B = 0.

Answer Answer:
(d)

Q.39. A plane electromagnetic wave propagating along x direction can have the following pairs of E and B
(a) Ex,By.
(b) Ey, Bz.
(c) Bx, Ey.
(d) None of these

Answer Answer:
(b)

Q.40. A charged particle oscillates about its mean equilibrium position with a frequency of 109 Hz. For producing electromagnetic waves which one is not true?
(a) They will have frequency of 109 Hz.
(b) They will have frequency of 2 × 109 Hz.
(c) They will have a wavelength of 0.3 m.
(d) They fall in the region of radio waves.

Answer Answer:
(b)

Q.41. The source of electromagnetic waves can be a charge
(a) moving with a constant velocity.
(b) moving in a circular orbit.
(c) at rest.
(d) falling in a magnetic field.

Answer Answer:
(b)

Q.42. One requires 11 eV of energy to dissociate a carbon monoxide molecule into carbon and oxygen atoms. The minimum frequency of the appropriate electromagnetic radiation to achieve the dissociation lies in [NCERT Exemplar]
(a) visible region.
(b) infrared region.
(c) ultraviolet region.
(d) microwave region.

Answer Answer:
(c)

Q.43. The ratio of contributions made by the electric field and magnetic field components to the intensity of an EM wave is [NCERT Exemplar]
(a) c : 1
(6) c² : 1
(c) 1 : 1
(d) √c : 1

Answer Answer:
(c)

Q.44. An electromagnetic wave travels in vacuum along z direction: E = (E{i + E2j) cos(kz – cot). Choose the correct options from the following: (c) The given electromagnetic field is circularly polarised.
(d) The given electromagnetic wave is unpolarised.

Answer Answer:
(a)

Q.45. Speed of electromagnetic wave related to electric field and magnetic field vector in vacuum. Answer Answer:
(a)

Q.46. An EM wave of intensity I falls on a surface kept in vacuum and exerts radiation pressure p on it. Which of the following is not true? [NCERT Exemplar]
(a) Radiation pressure is I/c if the wave is totally absorbed.
(b) Radiation pressure is I/c if the wave is totally reflected.
(c) Radiation pressure is 2I/c if the wave is totally reflected.
(d) Radiation pressure is in the range I/c < p < 2I/c for real surfaces.

Answer Answer:
(b)

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