CBSE Class 12 Physics Electromagnetic Waves MCQs Set B

Question. Electromagnetic wave consists of periodically oscillating electric and magnetic vectors
(a) in mutually perpendicular planes but vibrating with a phase difference of Π
(b) in mutually perpendicular planes but vibrating with a phase difference of Π/2
(c) in randomly oriented planes but vibrating in phase
(d) in mutually perpendicular planes but vibrating in phase

Answer : D

Question. A parallel plate capacitor of capacitance 20 mF 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 mA
(c) 60 mA, 60 mA
(d) 60 mA, zero

Answer : C

Question. A 100 W resistance and a capacitor of 100 W 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. 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. 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. 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. 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 in volt/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. If the magnetic field of a light wave oscillates parallel to y-axis and is given by By = Bm sin (kz – wt), the direction of wave travel and the axis along which the electric vector oscillates is :
(a) + ve y – axis, z– axis
(b) – ve z – axis, x– axis
(c) – ve x – axis, y– axis
(d) – ve x – axis, z– axis

Answer : B

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. 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. 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. Which wavelength of sun is used finally as electric energy?
(a) Radio waves
(b) Infra red waves
(c) Visible light
(d) Micro waves

Answer : B

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. 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. 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 the options

Answer : D

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

Answer : B

Question. Intensity of electromagnetic wave will be
(a) I = cμ₀B_O² / 2
(b) I = c∈₀B²₀ / 2
(c) I = B²₀ / cμ₀
(d) I = ∈²₀ / 2c∈₀

Answer : B

Question. The frequency 2 MHz belongs to
(a) visible light
(b) X-rays
(c) microwaves
(d) radiowaves

Answer : D

Question. The speed of electromagnetic wave in vacuum depends upon the source of radiation. It
(a) increases as we move from g-rays to radio waves
(b) decreases as we move from g-rays to radio waves
(c) is same for all of them
(d) None of the options

Answer : C

Question. The ionosphere
(a) reflects back radiowaves in the AM band
(b) reflects back radiowaves in the FM band
(c) absorbs radiowaves in the AM band
(d) absorbs radiowaves in the FM band

Answer : A

Question. In an electromagnetic wave
(a) power is transmitted along the magnetic field
(b) power is transmitted along the electric field
(c) power is equally transferred along the electric and magnetic fields
(d) power is transmitted in a direction perpendicular to both the fields

Answer : D

Question. The ozone layer absorbs radiation of wavelengths
(a) less than 3 × 10^–7 m
(b) more than 3 × 10^–7 m
(c) less than 3 × 10^–5 m
(d) more than 3 × 10^–5 m

Answer : A

Question. An electromagnetic wave going through vacuum is described by E = E₀ sin (kx – ϖt); B = B₀ sin (kx – ϖt). Which of the following equations is true?
(a) E0k = B0ϖ
(b) E0ϖ = B0k
(c) E0B0 = ϖk
(d) None of the options

Answer : A

Question. 10 cm is a wavelength corresponding to the spectrum of
(a) infrared rays
(b) ultra-violet rays
(c) microwaves
(d) g -rays

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. 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. According to Maxwell’s hypothesis, changing of electric field give rise to :
(a) magnetic field
(b) pressure gradient
(c) charge
(d) voltage

Answer : A

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. 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. The electric field of an electromagnetic wave in free space is given by E vector= 10cos(107 t + kx)jˆ V /m , where t and x are in seconds and metres respectively. It can be inferred that
(1) the wavelength l 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 : D

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. Which of the following rays are not electromagnetic waves ?
(a) X-rays
(b) g-rays
(c) b-rays
(d) heat rays

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. 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) g-rays
(d) infra-red radiations

Answer : B

Question. The electromagnetic waves travel with a velocity
(a) equal to velocity of sound
(b) equal to velocity of light
(c) less than velocity of light
(d) None of the options

Answer : B

Question. If λ = 10Å then it corresponds to
(a) infrared
(b) microwaves
(c) ultraviolet
(d) X-rays

Answer : D

Question. Radio waves and visible light in vacuum have
(a) same velocity but different wavelength
(b) continuous emission spectrum
(c) band absorption spectrum
(d) line emission spectrum

Answer : A

Question. Electromagnetic waves are transverse in nature is evident by
(a) polarization
(b) interference
(c) reflection
(d) diffraction

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. In an electromagnetic wave in free space the root mean square value of the electric field is Erms = 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. Which of the following are not electromagnetic waves?
(a) cosmic rays
(b) g-rays
(c) b-rays
(d) X-rays

Answer : C

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 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. If lv, lx and lm represent the wavelengths of visible light, X-rays and microwaves respectively, then
(a) lm > lx > lv
(b) lm > lv > lx
(c) lv > lx > lm
(d) lv > lm > lx.

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 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. Frequency of infrared wave is approximately:
(a) 1018 Hz
(b) 1014 Hz
(c) 109 Hz
(d) 1016 Hz

Answer : B

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

Answer : D

Question. An electromagnetic wave passes through space and its equation is given by E = E0 sin (ϖt – kx) where E is electric field. Energy density of electromagnetic wave in space is

Answer : A