CBSE Class 12 Physics Electromagnetic Waves MCQs Set D

Question: In an electromagnetic wave in free space the root mean square value of the electric field is E_rms = 6 V m^–1. The peak value of the magnetic field is
(a) 2.83 × 10^–8 T
(b) 0.70 × 10^–8 T
(c) 4.23 × 10^–8 T
(d) 1.41 × 10^–8 T
Answer: a

Question: Out of the following options which one can be used to produce a propagating electromagnetic wave ?
(a) A chargeless particle
(b) An accelerating charge
(c) A charge moving at constant velocity
(d) A stationary charge
Answer: b

Question: The ratio of contributions made by the electric field and magnetic field components to the intensity of an electromagnetic wave is (c = speed of electromagnetic waves)
(a) c : 1
(b) 1 : 1
(c) 1 : c
(d) 1 : c²
Answer: b

Question: 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 wave is divided equally between electric and magnetic 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: c

Question: Light with an average flux of 20 W/cm² falls on a non-reflecting surface at normal incidence having surface area 20 cm². The energy received by the surface during time span of 1 minute is
(a) 10 × 10³ J
(b) 12 × 10³ J
(c) 24 × 10³ J
(d) 48 × 10³ J
Answer: c

 Question: What is the cause of Green house effect?
(a) Infra-red rays
(b) Ultra violet rays
(c) X-rays
(d) Radio waves
Answer: a

Question: The electric and magnetic field of an electromagnetic wave are
(a) in opposite phase and perpendicular to each other
(b) in opposite phase and parallel to each other
(c) in phase and perpendicular to each other
(d) in phase and parallel to each other. 
Answer: c

Question: Light with an energy flux of 25 × 10⁴ W m^–2 falls on a perfectly reflecting surface at normal incidence. If the surface area is 15 cm², the average force exerted on the surface is
(a) 1.25 × 10^–6 N
(b) 2.50 × 10^–6 N
(c) 1.20 × 10^–6 N
(d) 3.0 × 10^–6 N
Answer: b

Question: We consider the radiation emitted by the human body. Which one of the following statements is true?
(a) The radiation emitted is in the infrared region.
(b) The radiation is emitted only during the day.
(c) The radiation is emitted during the summers and absorbed during the winters.
(d) The radiation emitted lies in the ultraviolet region and hence is not visible. 
Answer: a

Question: The electric field associated with an em wave in vacuum is given by [E = i ^{^} 40 cos (kz – 6 × 10⁸t)]; where E, z and t are involt/m, meter 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: a

Question: Which of the following rays are not electromagnetic waves ?
(a) X-rays
(b) Υ-rays
(c) β-rays
(d) heat rays 
Answer: c

 Question: For a transparent medium relative permeability and permittivity, μr and er are 1.0 and 1.44 respectively. The velocity of light in this medium would be
(a) 2.5 × 10⁸ m/s
(b) 3 × 10⁸ m/s
(c) 2.08 × 10⁸ m/s
(d) 4.32 × 10⁸ m/s
Answer: a

Question: The ratio of amplitude of magnetic field to the amplitude of electric field for an electromagnetic wave propagating in vacuum is equal to
(a) the speed of light in vacuum
(b) reciprocal of speed of light in vacuum
(c) the ratio of magnetic permeability to the electric susceptibility of vacuum
(d) unity.
Answer: b

Question: The frequency of electromagnetic wave, which best suited to observe a particle of radius 3 × 10^–4 cm is of the order of
(a) 10¹⁵
(b) 10¹⁴
(c) 10¹³
(d) 10¹²
Answer: b

Question: An electromagnetic wave of frequency u = 3.0 MHz passes from vacuum into a dielectric medium with relative permittivity e = 4.0. Then
(a) Wavelength is doubled and frequency becomes half.
(b) Wavelength is halved and frequency remains unchanged.
(c) Wavelength and frequency both remain unchanged.
(d) Wavelength is doubled and frequency unchanged
Answer: b

 Question: Ozone layer blocks the radiations of wavelength
(a) more than 3 × 10^–7 m
(b) equal to 3 × 10^–7 m
(c) less than 3 × 10^–7 m
(d) all of these
Answer: d

Question: The velocity of electromagnetic wave is parallel to
(a) B^→ × E^→
(b) E^→ × B^→
(c) E^→
(d) B^→
Answer: b

 Question: The frequency order for g-rays (B), X-rays (A),UV rays (C) is
(a) B > A > C
(b) A > B > C
(c) C > B > A
(d) A > C > B.
Answer: a

Question: An em wave is propagating in a medium with a velocity v^→ = v ^{^}i . The instantaneous oscillating electric field of this em wave is along +y axis. Then the direction of oscillating magnetic field of the em wave will be along
(a) –z direction
(b) +z direction
(c) –y direction
(d) –x direction
Answer: b

Question: Which of the following electromagnetic radiations have the longest wavelength ?
(a) X-rays
(b) Υ-rays
(c) Microwaves
(d) Radiowaves
Answer: d

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

Question:A parallel plate capacitor of capacitance 20 μF is being charged by a voltage source whose potential is changing at the rate of 3 V/s. The conduction current through the connecting wires, and the displacement current through the plates of the capacitor, would be, respectively
(a) zero, zero
(b) zero, 60 μA
(c) 60 μA, 60 μA
(d) 60 μA, zero
Answer: c

Question: A 100 Ω resistance and a capacitor of 100 Ω reactance are connected in series across a 220 V source. When the capacitor is 50% charged, the peak value of the displacement current is
(a) 2.2 A
(b) 11 A
(c) 4.4 A
(d) 11 √2 A
Answer: a

Question: Wavelength of light of frequency 100 Hz is
(a) 4 × 10⁶ m
(b) 3 × 10⁶ m
(c) 2 × 10⁶ m
(d) 5 × 10^–5 m 
Answer: b

Question: Which of the following electromagnetic radiations have the smaller wavelength?
(a) X-rays
(b) Υ-rays
(c) UV waves
(d) microwaves 
Answer: b

Question: The decreasing order of wavelength of infrared, microwave, ultraviolet and gamma rays is
(a) microwave, infrared, ultraviolet, gamma rays
(b) gamma rays, ultraviolet, infrared, microwaves
(c) microwaves, gamma rays, infrared, ultraviolet
(d) infrared, microwave, ultraviolet, gamma rays
Answer: a

Question: The energy of the em waves is of the order of 15 keV. To which part of the spectrum does it belong?
(a) Ultraviolet rays
(b) g-rays
(c) X-rays
(d) Infra-red rays
Answer: c

Question: Biological importance of ozone layer is
(a) it stops ultraviolet rays
(b) ozone layer reduces green house effect
(c) ozone layer reflects radio waves
(d) ozone layer controls O₂/H₂ ratio in atmosphere.
Answer: a

Question: A signal emitted by an antenna from a certain point can be received at another point of the surface in the form of
(a) sky wave
(b) ground wave
(c) sea wave
(d) both (a) and (b)
Answer: d

Question: The structure of solids is investigated by using
(a) cosmic rays
(b) X-rays
(c) Υ-rays
(d) infra-red radiations
Answer: b

Question: If λ_v, l_x and λm represent the wavelengths of visible light, X-rays and microwaves respectively, then
(a) λ_m > λ_x > λ_v
(b) λ_m > λ_v > λ_x
(c) λ_v > λ_x > λ_m
(d) λ_v > λ_m > λ_x. 
Answer: b

Question: The electric field of an electromagnetic wave in free space is given by E^→ = 10 cos (10⁷ t + kx)jˆ V /m , where t and x are in seconds and metres respectively. It can be inferred that
(1) the wavelength λ is 188.4 m.
(2) the wave number k is 0.33 rad/m.
(3) the wave amplitude is 10 V/m.
(4) the wave is propagating along +x direction.
Which one of the following pairs of statements is correct?
(a) (3) and (4)
(b) (1) and (2)
(c) (2) and (3)
(d) (1) and (3)
Answer: b