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1. Electromagnetic waves were first predicted theoretically by:
A. Heinrich Hertz
B. James Clerk Maxwell
C. Michael Faraday
D. Albert Einstein
Answer: B. James Clerk Maxwell
Explanation: James Clerk Maxwell theoretically predicted the existence of electromagnetic waves through his set of equations unifying electricity and magnetism; Hertz later verified them experimentally.
2. Electromagnetic waves are:
A. Longitudinal waves
B. Transverse waves
C. Neither longitudinal nor transverse
D. Mechanical waves
Answer: B. Transverse waves
Explanation: In an EM wave, the electric and magnetic field vectors oscillate perpendicular to each other and to the direction of wave propagation, making it a transverse wave.
3. The speed of electromagnetic waves in vacuum is given by:
A. c = μ0ε0
B. c = 1/√(μ0ε0)
C. c = √(μ0ε0)
D. c = μ0/ε0
Answer: B. c = 1/√(μ0ε0)
Explanation: Maxwell showed that EM waves travel through vacuum at speed c = 1/√(μ0ε0), which matches the experimentally measured speed of light.
4. Electromagnetic waves, unlike sound waves, can travel through:
A. Only solids
B. Only gases
C. Vacuum, as they do not require a material medium
D. Only liquids
Answer: C. Vacuum, as they do not require a material medium
Explanation: EM waves consist of oscillating electric and magnetic fields that sustain each other and do not need a material medium, so they can propagate through vacuum.
5. The concept of 'displacement current' was introduced by Maxwell to:
A. Replace conduction current entirely
B. Make Ampere's circuital law consistent in situations with changing electric fields
C. Explain electromagnetic induction
D. Define electric charge
Answer: B. Make Ampere's circuital law consistent in situations with changing electric fields
Explanation: Maxwell added the displacement current term (ε0 dΦE/dt) to Ampere's law to account for changing electric flux, such as between capacitor plates, making the law self-consistent.
6. Arrange in order of increasing wavelength: X-rays, radio waves, visible light, gamma rays.
A. Radio waves, visible light, X-rays, gamma rays
B. Gamma rays, X-rays, visible light, radio waves
C. Visible light, gamma rays, X-rays, radio waves
D. X-rays, gamma rays, radio waves, visible light
Answer: B. Gamma rays, X-rays, visible light, radio waves
Explanation: In the electromagnetic spectrum, gamma rays have the shortest wavelength, followed by X-rays, then visible light, and radio waves have the longest wavelength.
7. Which part of the electromagnetic spectrum has the longest wavelength?
A. Gamma rays
B. X-rays
C. Ultraviolet rays
D. Radio waves
Answer: D. Radio waves
Explanation: Radio waves occupy the low-frequency, long-wavelength end of the electromagnetic spectrum, used commonly for broadcasting and communication.
8. Microwaves are commonly used in:
A. Radar systems and microwave ovens
B. Medical imaging of bones
C. Sterilizing surgical instruments
D. Photography of stars
Answer: A. Radar systems and microwave ovens
Explanation: Microwaves, due to their short wavelength and ability to be focused into beams, are used in radar for detecting objects and in microwave ovens for heating food.
9. Infrared radiation is mainly associated with:
A. Nuclear transitions
B. Heat radiation and is used in remote controls
C. High-energy electron transitions in atoms
D. Radioactive decay
Answer: B. Heat radiation and is used in remote controls
Explanation: Infrared waves are produced by molecular vibrations and warm bodies, are felt as heat, and are commonly used in remote controls, thermal imaging, and greenhouses.
10. The approximate wavelength range of visible light is:
A. 1 nm to 10 nm
B. 400 nm to 700 nm
C. 10 μm to 100 μm
D. 1 mm to 10 mm
Answer: B. 400 nm to 700 nm
Explanation: Visible light, the narrow band of the EM spectrum detectable by the human eye, spans roughly from 400 nm (violet) to 700 nm (red).
11. Ultraviolet radiation is used for:
A. Sterilizing water and surgical instruments
B. Detecting broken bones
C. Radio communication
D. Cooking food
Answer: A. Sterilizing water and surgical instruments
Explanation: UV radiation has enough energy to kill bacteria and microorganisms, so it is widely used for sterilization of water, air, and medical equipment.
12. X-rays are produced when:
A. Radioactive nuclei decay
B. High-energy electrons are suddenly decelerated upon striking a metal target
C. Molecules rotate and vibrate
D. Atoms absorb infrared radiation
Answer: B. High-energy electrons are suddenly decelerated upon striking a metal target
Explanation: X-rays are generated when fast-moving electrons are abruptly stopped by a heavy metal target, converting their kinetic energy into high-frequency electromagnetic radiation.
13. Gamma rays originate from:
A. Changes in outer electron energy levels
B. Transitions within atomic nuclei
C. Oscillating electric circuits
D. Thermal vibrations of molecules
Answer: B. Transitions within atomic nuclei
Explanation: Gamma rays are emitted during nuclear transitions, such as radioactive decay, when a nucleus moves from a higher to a lower energy state, giving them very high frequency and energy.
14. Electromagnetic waves carry:
A. Only energy, no momentum
B. Only momentum, no energy
C. Both energy and momentum
D. Neither energy nor momentum
Answer: C. Both energy and momentum
Explanation: Electromagnetic waves transport both energy and momentum through space, which is evidenced by phenomena such as radiation pressure on illuminated surfaces.
15. The Poynting vector in an electromagnetic wave represents:
A. The direction and rate of energy transfer per unit area
B. The electric field direction only
C. The magnetic field magnitude only
D. The wavelength of the wave
Answer: A. The direction and rate of energy transfer per unit area
Explanation: The Poynting vector, S = E × B/μ0, points along the direction of wave propagation and gives the rate of energy flow per unit area carried by the EM wave.
16. In an electromagnetic wave travelling in vacuum, the ratio of the amplitudes of the electric field (E0) to the magnetic field (B0) equals:
A. 1
B. c (speed of light)
C. 1/c
D. c²
Answer: B. c (speed of light)
Explanation: For an EM wave in vacuum, the electric and magnetic field amplitudes are related by E0/B0 = c, the speed of light, as required by Maxwell's equations.
17. Amplitude modulation, used in radio broadcasting, involves varying:
A. The frequency of the carrier wave
B. The amplitude of the carrier wave in accordance with the signal
C. The phase of the carrier wave only
D. The wavelength of the carrier medium
Answer: B. The amplitude of the carrier wave in accordance with the signal
Explanation: In amplitude modulation (AM), the amplitude of a high-frequency carrier wave is varied in proportion to the instantaneous amplitude of the audio signal being transmitted.
18. The ozone layer in the atmosphere is important because it:
A. Reflects radio waves back to Earth
B. Absorbs most of the harmful ultraviolet radiation from the Sun
C. Blocks visible light
D. Produces gamma rays
Answer: B. Absorbs most of the harmful ultraviolet radiation from the Sun
Explanation: Ozone in the stratosphere absorbs a large portion of the Sun's ultraviolet radiation, protecting living organisms on Earth from its harmful biological effects.
19. The displacement current density is mathematically expressed as:
A. Jd = ε0 (dE/dt)
B. Jd = σE
C. Jd = μ0H
D. Jd = ε0 μ0
Answer: A. Jd = ε0 (dE/dt)
Explanation: Displacement current density arises from a time-varying electric field and is given by Jd = ε0 (dE/dt), which Maxwell added to complete Ampere's circuital law.
20. Which property is common to all electromagnetic waves in vacuum, regardless of their frequency?
A. Same wavelength
B. Same speed (c)
C. Same energy
D. Same amplitude
Answer: B. Same speed (c)
Explanation: All electromagnetic waves, irrespective of their frequency or wavelength, travel through vacuum at the same speed c ≈ 3 × 10⁸ m/s.
This page contains a chapter-wise objective practice set. Try the questions without checking the answer first, then use the explanation to understand the point you missed. The current page contains 20 questions, so you can practise in smaller sets when you are revising.
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After the practice set, write down the question numbers you missed and revise only those concepts once more. This makes MCQ practice more useful than repeatedly attempting the same questions without reviewing mistakes.
