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NCERT Solutions for class 12 Physics | Chapter 8 - Electromagnetic Waves

(1) A plane electromagnetic wave is incident on a material surface. The wave delivers momentum p and energy E. Then
[A] p ≠ 0, E ≠ 0
[B] p = 0, E = 0
[C] p = 0, E ≠ 0
[D] p ≠ 0, E = 0
Answer: p ≠ 0, E = 0
(2) 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: in phase and perpendicular to each other

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(3) The electromagnetic wave used to treat muscular pain is:
[A] VHF radiowaves
[B] X-rays
[C] Ultraviolet rays
[D] Infrared radiations
Answer: Infrared radiations
(4) A beam of light travelling along x-axis is described by the magnetic field Bz = 5 X 10-7T sinω(t – x/c) If a beam of alpha particle moves in this filed along y – axis with a speed of 3 X 107 m/s, then the maximum electric and magnetic forces experienced by it will be:
[A] 4.80 X 10-10N and 4.8 X 10-12N respectively.
[B] 4.80 X 10-17N and 4.8 X 10-18N respectively.
[C] 2.80 X 10-5N and 2.1 X 10-7N respectively.
[D] 1.5 X 10-5N and 4.1 X 10-7N respectively.
Answer: 4.80 X 10-17N and 4.8 X 10-18N respectively.
(5) An electromagnetic wave propagating along north has its electric field vector upwards. Its magnetic field vector point towards
[A] north
[B] east
[C] west
[D] downwards
Answer: east
(6) According to Maxwell’s hypothesis, a changing electric field gives rise to
[A] an e.m.f
[B] electric displacement current
[C] magnetic field
[D] pressure gradient
Answer: magnetic field
(7) Which of the following has/have zero average value in a plane electromagnetic wave?
[A] Both magnetic and electric field
[B] Electric field only
[C] Magnetic energy
[D] Electric energy
Answer: Both magnetic and electric field
(8) Select the wrong statement. EM waves
[A] are transverse in nature.
[B] travel in free space at a speed of light.
[C] are produced by accelerating charges.
[D] travel in all media with same speed.
Answer: travel in all media with same speed.
(9) An electron oscillating with a frequency of 3 × 106 Hz, would generate
[A] X-rays
[B] ultraviolet rays
[C] radio waves
[D] microwaves
Answer: radio waves
(10) Displacement current goes through the gap between the plates of a capacitor when the charge on the capacitor
[A] is changing with time
[B] decreases
[C] does not change
[D] decreases to zero
Answer: is changing with time
(11) If a variable frequency ac source connected to a capacitor then with decrease in frequency, the displacement current will
[A] increase
[B] decrease
[C] remain constant
[D] first decrease then increase
Answer: decrease
(12) The current in the electric circuit which arises due to flow of electrons in the connecting wires of the circuit in a defined closed path is called
[A] alternating current
[B] direct current
[C] conduction current
[D] displacement current
Answer: conduction current
(13) The polarisation of electromagnetic wave is in
[A] the directions of electric and magnetic field
[B] the directions of electric field
[C] the direction of magnetic field
[D] can not be polarized
Answer: the directions of electric field
(14) The electromagnetic waves do not transport
[A] energy
[B] charge
[C] momentum
[D] information
Answer: charge
(15) The electromagnetic radiation used in food processing sterilizing agent is
[A] microwaves
[B] UV rays
[C] gamma rays
[D] radio waves
Answer: UV rays
(16) The waves used in telecommunication are
[A] IR
[B] UV
[C] Microwave
[D] Cosmic rays
Answer: Microwave
(17) What is the cause of “Green house effect”?
[A] Infrared rays
[B] Ultraviolet rays
[C] X-rays
[D] Radio waves
Answer: Infrared rays
(18) Radio waves do not penetrate in the band of
[A] ionosphere
[B] mesosphere
[C] troposphere
[D] stratosphere
Answer: ionosphere
(19) The wavelength of X-ray is of the order of
[A] 1 metre
[B] 1 cm
[C] 1 micron
[D] 1 angstrom
Answer: 1 angstrom
(20) The range of wavelength of visible light is
[A] 50 Å to 500 Å
[B] 4000 Å to 8000 Å
[C] 9000 Å to 25,000 Å
[D] 15,000 Å to 25,000 Å
Answer: 4000 Å to 8000 Å

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