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Electromagnetic Waves — NEET & JEE MCQs with answers
Maxwell showed that a time-varying electric field produces a magnetic field (displacement current), just as a time-varying magnetic field produces an electric field. This led to the prediction of electromagnetic (EM) waves travelling at the speed of light $c = 1/\sqrt{\mu0\varepsilon0} \approx 3\times10^8$ m/s, unifying electricity, magnetism and light. The chapter covers displacement current, the nature of EM waves, and the electromagnetic spectrum.
Topics in this chapter
- 8.1 Introduction
- 8.2 Displacement Current
- 8.3 Electromagnetic Waves
- 8.4 Electromagnetic Spectrum
Free practice questions
The electric field amplitude of an electromagnetic wave in vacuum is $E_0=4.5$ V/m. Using $c=3\times10^{8}$ m/s, the magnetic field amplitude is:
- A. $1.5\times10^{-7}$ T
- B. $1.5\times10^{-8}$ T
- C. $1.5\times10^{-9}$ T
- D. $1.35\times10^{9}$ T
Show answer
Answer: B — $B_0=E_0/c=4.5/(3\times10^{8})=1.5\times10^{-8}$ T.
NCERT Class 12 Physics Ch 8 ("B0 = (E0/c)")
The electromagnetic spectrum extends, in order of increasing wavelength, up to a longest wavelength of about $10^{6}$ m. Which band corresponds to this longest-wavelength end?
- A. Microwaves
- B. Gamma rays
- C. Infrared waves
- D. Radio waves
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Answer: D — The spectrum runs by increasing wavelength from about $10^{-12}$ m (gamma rays) to about $10^{6}$ m. Radio waves occupy the long-wavelength extreme, so the $\sim 10^{6}$ m end corresponds to radio waves.
NCERT Class 12 Physics Ch 8 ("in order of increasing wavelength")
Whose technological use of electromagnetic waves led to the revolution in communication?
- A. Marconi
- B. Coulomb
- C. Ampere
- D. Gauss
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Answer: A — The technological use by Marconi and others led to the revolution in communication.
NCERT Class 12 Physics Ch 8 ("revolution in communication")
If the displacement current in a region is $1\times10^{-2}$ A, then using $i_d=\varepsilon_0\,d\Phi_E/dt$ the rate of change of electric flux is:
- A. $1.13\times10^{8}$ V·m/s
- B. $1.13\times10^{10}$ V·m/s
- C. $2.26\times10^{9}$ V·m/s
- D. $1.13\times10^{9}$ V·m/s
Show answer
Answer: D — $\dfrac{d\Phi_E}{dt}=\dfrac{i_d}{\varepsilon_0}=\dfrac{1\times10^{-2}}{8.85\times10^{-12}}=1.13\times10^{9}$ V·m/s.
NCERT Class 12 Physics Ch 8 ("This implies that for consistency,")
According to NCERT, which celestial body is described as an important source of ultraviolet light?
- A. Comets
- B. The Sun
- C. Distant radio galaxies
- D. The Moon
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Answer: B — The text explicitly states that the sun is an important source of ultraviolet light, most of which is fortunately absorbed in the ozone layer.
NCERT Class 12 Physics Ch 8 ("The sun is an important source of ultraviolet")
The fact that the electric field of the capacitor is perpendicular to the plates while the magnetic field it produces is along a circle parallel to the plates shows that $\mathbf{B}$ and $\mathbf{E}$ are perpendicular. NCERT states this is:
- A. True only in a material medium, not in vacuum
- B. A general feature of electromagnetic waves
- C. True only for longitudinal waves
- D. A special case that only applies to a capacitor
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Answer: B — After noting that in the capacitor case $\mathbf{B}$ and $\mathbf{E}$ are perpendicular, NCERT states "This is a general feature" — mutual perpendicularity of the fields holds for all electromagnetic waves.
NCERT Class 12 Physics Ch 8 ("This is a general feature")
The electric flux between the plates of a charging capacitor changes at a rate of $3\times10^{7}$ V·m/s. The displacement current between the plates is:
- A. $5.31\times10^{-4}$ A
- B. $2.65\times10^{-5}$ A
- C. $2.66\times10^{-3}$ A
- D. $2.65\times10^{-4}$ A
Show answer
Answer: D — $i_d=\varepsilon_0\dfrac{d\Phi_E}{dt}=8.85\times10^{-12}\times 3\times10^{7}=2.65\times10^{-4}$ A.
NCERT Class 12 Physics Ch 8 ("This implies that for consistency,")
Maxwell (1831-1879) argued that the converse of Faraday's result 'was indeed the case'. This converse states that:
- A. A magnetic field can never make an electric field
- B. A time-varying electric field generates a magnetic field
- C. Charges cannot produce electric fields
- D. A steady current generates no magnetic field
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Answer: B — The converse is that a time-varying electric field generates a magnetic field.
NCERT Class 12 Physics Ch 8 ("argued that this was indeed the case")
A stationary electric charge produces only:
- A. Electrostatic fields
- B. Time-varying magnetic fields
- C. Electromagnetic waves
- D. Oscillating electric and magnetic fields
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Answer: A — NCERT: neither stationary charges nor charges in uniform motion can be sources of electromagnetic waves; the former produces only electrostatic fields.
NCERT Class 12 Physics Ch 8 ("The former produces only electrostatic fields")
A capacitor is being charged by a constant conduction current of $1.5\times10^{-1}$ A in the connecting wires. The displacement current between the plates is:
- A. $1.5\times10^{-1}$ A
- B. $7.5\times10^{-2}$ A
- C. $3\times10^{-1}$ A
- D. $0$ A
Show answer
Answer: A — For a charging capacitor the displacement current equals the conduction current in magnitude, so $i_d=i_c=1.5\times10^{-1}$ A.
NCERT Class 12 Physics Ch 8 ("so that id = i")
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