CBSE Class 12 Physics Electromagnetic Waves MCQs Set E

Question. 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
(a) visible region
(b) infrared region
(c) ultraviolet region
(d) microwave region

Answer: C

Question. A plane electromagnetic wave travelling along X-axis has a wavelength 10.0 mm. The electric field points along Y-direction and has peak value of 30 V/m. Then the magnetic field in terms of x in metre and t in second may be expressed as
(a) 30 sin 200π ( ct – x)
(b) 10^–7 sin 200π (ct– x)
(c) 30 sin 2π/10 (ct – x)
(d) 10^-7 sin 2π/10 (ct – x)

Answer: B

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

Answer: B

Question. A linearly polarised electromagnetic wave given as E = E₀ i 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
(a) E_r = - E₀ i cos (kz – ωt) 
(b) E_r = E₀ i cos (kz + ωt) 
(c) E_r = - E₀ i cos (kz + ωt) 
(d) E_r = E₀ i sin (kz – ωt) 

Answer: B

Question. 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
(a) 36 × 10^–5 kg m/s
(b) 36 × 10^–4 kg m/s
(c) 108 × 10⁴ kg m/s
(d) 1.08 × 10⁷ kg m/s

Answer: B

Question. A 100 Ω resistance and a capacitor of 100 Ω reactance are connected in series across a 22 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. An LC circuit contains inductance L = 1mH and capacitance C = 0.01 μF. The wavelength of the electromagnetic wave generated is nearly
(a) 0.5 m
(b) 5 m
(c) 30 m
(d) 188 m

Answer: D

Question. The radiowaves of wavelength 360 m are transmitted from a transmitter. The inductance of the coil which must be connected with capacitor of capacitance 3.6 μF in a resonant circuit to receive these waves will be nearly
(a) 10³ H
(b) 10² H
(c) 10^–4 H
(d) 10^–8 H

Answer: D

Question. What is the amplitude of electric field produced by radiation coming from a 100 W bulb at a distance of 4 m? The efficiency of bulb is 3.14% and it may be assumed as a point source.
(a) 2.42 V/m
(b) 3.43 V/m
(c) 4.2 × 10⁴ V/m
(d) 14 × 10⁴ V/m

Answer: B

Question. 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
(a) E/2
(b) 2E
(c) E/√2
(d) √2E

Answer: C

Question. If E and B represent electric and magnetic field vectors of the electromagnetic wave, the direction of propagation of electromagnetic wave is along
(a) E
(b) B
(c) B × E
(d) E × B

Answer: D

Question. If we want to produce electromagnetic waves of wavelength 500 km by an oscillating charge; then frequency of oscillating charge must be
(a) 600 Hz
(b) 500 Hz
(c) 167 Hz
(d) 15 Hz

Answer: A

Question. Electromagnetic waves travelling in a medium having relative permeability μ_r = 1.3 and relative permittivity ∈_r = 2.14 . The speed of electromagnetic waves in medium must be
(a) 1.8 × 10⁸ m/s
(b) 1.8 × 10⁴ m/s
(c) 1.8 × 10⁶ m/s
(d) 1.8 × 10² m/s

Answer: A

Question. Electromagnetic waves travelling in a medium has speed 2 × 10⁸ m/s. If the relative permeability is 1, then the relative permittivity of medium must be
(a) 2
(b) 2.25
(c) 2.5
(d) 1.5

Answer: B

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

Answer: C

Question. An electromagnetic 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
(a) 1/r³
(b) 1/r²
(c) 1/r
(d) remains constant

Answer: C

Question. A plane electromagnetic wave of energy U is reflected from the surface. Then the momentum transferred by electromagnetic wave to the surface is
(a) 0
(b) U/c
(c) 2U/c
(d) U/2c

Answer: C

Question. The rms value of the electric field of light coming from the sun is 720 N/C. The average total energy density of the electromagnetic wave is :
(a) 4.58 × 10^–6 J/m³
(b) 6.37 × 10^–9 J/m³
(c) 1.35 × 10^–12 J/m³
(d) 3.3 × 10^–3 J/m³

Answer: A

Question. A plane electromagnetic wave propagating along x direction can have the following pairs of E and B
(a) E_x, B_y
(b) E_y, B_z
(c) B_x, E_y
(d) E_z, B_y

Answer: B, D

Fill in the Blanks

Question. In case of electromagnetic wave, the vibrating electric field vector (^→E ) and magnetic field vector (^→B) are mutually perpendicular to each other and both are perpendicular to the direction of _______.
Answer: propagation

Question. The current which comes into play in the region, whenever the electric field and hence the electric flux is changing with time is called _______.
Answer: displacement current

Question. The orderly distribution of electromagnetic radiations according to their frequency or wavelength is called _______.
Answer: electromagnetic spectrum

Question. The displacement current is precisely equal to the conduction current, when the two are present in different parts of the circuit. These currents are individually discontinuous, but the two currents together posses the property of _______ through any closed circuit.
Answer: continuity

Question. Electromagnetic wave is _______ in nature as the electric and magnetic fields are perpendicular to each other and to the direction of propagation of the wave.
Answer: transverse

Question. Electromagnetic waves are not _______ by electric and magnetic waves.
Answer: deflected

Question. The _______ of electromagnetic waves does not change when it goes from one medium to another but its wavelength changes.
Answer: frequency

Question. _______ particles radiate electromagnetic waves.
Answer: Accelerated charged

Question. The shortest wavelength radio waves are called _______.
Answer: micro-waves

Question. Ozone layer in the atmosphere plays a protective role, and hence its depletion by _____________  gas is a matter of international concern.
Answer: chlorofluorocarbons (CFCs)