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Electromagnetic Radiation – MCQs with
Answers & Explanations
Section 1: Fundamentals of Electromagnetic Radiation
Q1. Electromagnetic waves are best described as:
a) Longitudinal waves of electric fields only
b) Transverse waves of electric and magnetic fields
c) Sound waves propagating in space
d) Mechanical waves requiring a medium
Answer: b) Transverse waves of electric and magnetic fields
Explanation: EM waves consist of oscillating electric and magnetic fields that are mutually
perpendicular and also perpendicular to the direction of propagation. They are transverse in
nature and require no medium.
Q2. The speed of an electromagnetic wave in vacuum is approximately:
a) 3 × 10³ m/s
b) 3 × 10⁵ m/s
c) 3 × 10⁸ m/s
d) 3 × 10¹⁰ m/s
Answer: c) 3 × 10⁸ m/s
Explanation: The speed of light in vacuum is defined as 299,792,458 m/s, often approximated
as 3×108 m/s3 \times 10^8 \, \text{m/s}3×108m/s.
Q3. In electromagnetic waves, the electric field (E) and magnetic field (B) are related by:
a) E=BE = BE=B
b) E=cBE = cBE=cB
c) E=BcE = \dfrac{B}{c}E=cB
d) E=c2BE = c^2BE=c2B
Answer: b) E=cBE = cBE=cB
Explanation: In vacuum, the magnitudes of electric and magnetic fields follow E=cBE =
cBE=cB, where ccc is the speed of light.
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Q4. Which of the following is a property of electromagnetic waves?
a) They are deflected by electric fields only
b) They are deflected by magnetic fields only
c) They are not deflected by electric or magnetic fields
d) They require particles as carriers
Answer: c) They are not deflected by electric or magnetic fields
Explanation: EM waves are uncharged and hence not deflected by external static electric or
magnetic fields as whole entities.
Q5. The wave equation for EM radiation in free space is:
a) ∇2E=μ0σE\nabla^2 E = \mu_0 \sigma E∇2E=μ0σE
b) ∇2E=1c2∂2E∂t2\nabla^2 E = \dfrac{1}{c^2} \dfrac{\partial^2 E}{\partial t^2}∇2E=c21
∂t2∂2E
c) E=IRE = IRE=IR
d) ∇⋅B≠0\nabla \cdot B \neq 0∇⋅B=0
Answer: b) ∇2E=1c2∂2E∂t2\nabla^2 E = \dfrac{1}{c^2} \dfrac{\partial^2 E}{\partial t^2}∇2E=c21∂t2∂2E
Explanation: Derived from Maxwell’s equations, the wave equation in free space links the Laplacian of
the field to the second time derivative, showing wave-like behavior.
Section 2: Spectrum of Electromagnetic Radiation
Q6. Which radiation has the shortest wavelength?
a) Radio waves
b) Infrared radiation
c) Gamma rays
d) Microwaves
Answer: c) Gamma rays
Explanation: Gamma rays fall at the extreme high-frequency, short-wavelength end of the
spectrum (< 10⁻¹² m).
Q7. The electromagnetic spectrum in order of increasing frequency is:
a) Radio → Microwaves → IR → Visible → UV → X-rays → Gamma
b) Gamma → X-rays → UV → Visible → IR → Microwaves → Radio
c) X-rays → Gamma → UV → Visible → IR → Microwaves → Radio
d) Radio → Visible → Microwaves → IR → Gamma → X-rays → UV
Answer: a) Radio → Microwaves → IR → Visible → UV → X-rays → Gamma
Explanation: This is the correct ascending order of frequency and energy.
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Q8. Which statement about visible light is correct?
a) It occupies the largest portion of the EM spectrum
b) It is between infrared and ultraviolet radiation
c) Its wavelength range is 1 mm to 1 cm
d) Human eyes are sensitive to its polarization only
Answer: b) It is between infrared and ultraviolet radiation
Explanation: Visible light spans wavelengths ~400–700 nm, lying between infrared (700 nm+)
and ultraviolet (<400 nm).
Q9. Ultraviolet radiation has higher energy photons than:
a) Gamma rays
b) X-rays
c) Visible light
d) Cosmic rays
Answer: c) Visible light
Explanation: UV photons have shorter wavelengths (higher energies) than visible light, but less
energy than X-rays or gamma rays.
Q10. Which radiation is often used in medical imaging (CT scans)?
a) Infrared
b) Radio waves
c) X-rays
d) Gamma rays
Answer: c) X-rays
Explanation: X-rays penetrate body tissues differentially, making them highly useful for
medical diagnostics.
Section 3: Energy and Photon Concept
Q11. The energy of a photon is:
a) Directly proportional to its wavelength
b) Directly proportional to its frequency
c) Independent of frequency
d) Inversely proportional to frequency
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Answer: b) Directly proportional to its frequency
Explanation: E=hνE = h\nuE=hν. Energy increases with frequency.
Q12. A photon of wavelength 500 nm has energy (approximately):
a) 2.5 eV
b) 4 eV
c) 10 eV
d) 0.1 eV
Answer: a) 2.5 eV
Explanation: E=hc/λE = hc/\lambdaE=hc/λ. Using h=4.14×10−15 eV\cdotpsh = 4.14 \times
10^{-15}\,\text{eV·s}h=4.14×10−15eV\cdotps, c=3×108c = 3\times10^8c=3×108, λ = 500 nm
→ ≈2.5 eV\approx 2.5 \,\text{eV}≈2.5eV.
Q13. Which radiation has photons of highest energy?
a) X-rays
b) Visible light
c) Radio waves
d) Gamma rays
Answer: d) Gamma rays
Explanation: Gamma rays have the highest frequency, so by E=hνE = h\nuE=hν, they have the
most energetic photons.
Q14. Doubling the frequency of light will:
a) Double its wavelength
b) Halve its photon energy
c) Double its photon energy
d) Not affect its energy
Answer: c) Double its photon energy
Explanation: Photon energy is directly proportional to frequency.
Q15. Planck’s constant has units of:
a) J·m
b) J·Hz
c) J·s
d) N·s
Electromagnetic Radiation – MCQs with
Answers & Explanations
Section 1: Fundamentals of Electromagnetic Radiation
Q1. Electromagnetic waves are best described as:
a) Longitudinal waves of electric fields only
b) Transverse waves of electric and magnetic fields
c) Sound waves propagating in space
d) Mechanical waves requiring a medium
Answer: b) Transverse waves of electric and magnetic fields
Explanation: EM waves consist of oscillating electric and magnetic fields that are mutually
perpendicular and also perpendicular to the direction of propagation. They are transverse in
nature and require no medium.
Q2. The speed of an electromagnetic wave in vacuum is approximately:
a) 3 × 10³ m/s
b) 3 × 10⁵ m/s
c) 3 × 10⁸ m/s
d) 3 × 10¹⁰ m/s
Answer: c) 3 × 10⁸ m/s
Explanation: The speed of light in vacuum is defined as 299,792,458 m/s, often approximated
as 3×108 m/s3 \times 10^8 \, \text{m/s}3×108m/s.
Q3. In electromagnetic waves, the electric field (E) and magnetic field (B) are related by:
a) E=BE = BE=B
b) E=cBE = cBE=cB
c) E=BcE = \dfrac{B}{c}E=cB
d) E=c2BE = c^2BE=c2B
Answer: b) E=cBE = cBE=cB
Explanation: In vacuum, the magnitudes of electric and magnetic fields follow E=cBE =
cBE=cB, where ccc is the speed of light.
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Q4. Which of the following is a property of electromagnetic waves?
a) They are deflected by electric fields only
b) They are deflected by magnetic fields only
c) They are not deflected by electric or magnetic fields
d) They require particles as carriers
Answer: c) They are not deflected by electric or magnetic fields
Explanation: EM waves are uncharged and hence not deflected by external static electric or
magnetic fields as whole entities.
Q5. The wave equation for EM radiation in free space is:
a) ∇2E=μ0σE\nabla^2 E = \mu_0 \sigma E∇2E=μ0σE
b) ∇2E=1c2∂2E∂t2\nabla^2 E = \dfrac{1}{c^2} \dfrac{\partial^2 E}{\partial t^2}∇2E=c21
∂t2∂2E
c) E=IRE = IRE=IR
d) ∇⋅B≠0\nabla \cdot B \neq 0∇⋅B=0
Answer: b) ∇2E=1c2∂2E∂t2\nabla^2 E = \dfrac{1}{c^2} \dfrac{\partial^2 E}{\partial t^2}∇2E=c21∂t2∂2E
Explanation: Derived from Maxwell’s equations, the wave equation in free space links the Laplacian of
the field to the second time derivative, showing wave-like behavior.
Section 2: Spectrum of Electromagnetic Radiation
Q6. Which radiation has the shortest wavelength?
a) Radio waves
b) Infrared radiation
c) Gamma rays
d) Microwaves
Answer: c) Gamma rays
Explanation: Gamma rays fall at the extreme high-frequency, short-wavelength end of the
spectrum (< 10⁻¹² m).
Q7. The electromagnetic spectrum in order of increasing frequency is:
a) Radio → Microwaves → IR → Visible → UV → X-rays → Gamma
b) Gamma → X-rays → UV → Visible → IR → Microwaves → Radio
c) X-rays → Gamma → UV → Visible → IR → Microwaves → Radio
d) Radio → Visible → Microwaves → IR → Gamma → X-rays → UV
Answer: a) Radio → Microwaves → IR → Visible → UV → X-rays → Gamma
Explanation: This is the correct ascending order of frequency and energy.
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Q8. Which statement about visible light is correct?
a) It occupies the largest portion of the EM spectrum
b) It is between infrared and ultraviolet radiation
c) Its wavelength range is 1 mm to 1 cm
d) Human eyes are sensitive to its polarization only
Answer: b) It is between infrared and ultraviolet radiation
Explanation: Visible light spans wavelengths ~400–700 nm, lying between infrared (700 nm+)
and ultraviolet (<400 nm).
Q9. Ultraviolet radiation has higher energy photons than:
a) Gamma rays
b) X-rays
c) Visible light
d) Cosmic rays
Answer: c) Visible light
Explanation: UV photons have shorter wavelengths (higher energies) than visible light, but less
energy than X-rays or gamma rays.
Q10. Which radiation is often used in medical imaging (CT scans)?
a) Infrared
b) Radio waves
c) X-rays
d) Gamma rays
Answer: c) X-rays
Explanation: X-rays penetrate body tissues differentially, making them highly useful for
medical diagnostics.
Section 3: Energy and Photon Concept
Q11. The energy of a photon is:
a) Directly proportional to its wavelength
b) Directly proportional to its frequency
c) Independent of frequency
d) Inversely proportional to frequency
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Answer: b) Directly proportional to its frequency
Explanation: E=hνE = h\nuE=hν. Energy increases with frequency.
Q12. A photon of wavelength 500 nm has energy (approximately):
a) 2.5 eV
b) 4 eV
c) 10 eV
d) 0.1 eV
Answer: a) 2.5 eV
Explanation: E=hc/λE = hc/\lambdaE=hc/λ. Using h=4.14×10−15 eV\cdotpsh = 4.14 \times
10^{-15}\,\text{eV·s}h=4.14×10−15eV\cdotps, c=3×108c = 3\times10^8c=3×108, λ = 500 nm
→ ≈2.5 eV\approx 2.5 \,\text{eV}≈2.5eV.
Q13. Which radiation has photons of highest energy?
a) X-rays
b) Visible light
c) Radio waves
d) Gamma rays
Answer: d) Gamma rays
Explanation: Gamma rays have the highest frequency, so by E=hνE = h\nuE=hν, they have the
most energetic photons.
Q14. Doubling the frequency of light will:
a) Double its wavelength
b) Halve its photon energy
c) Double its photon energy
d) Not affect its energy
Answer: c) Double its photon energy
Explanation: Photon energy is directly proportional to frequency.
Q15. Planck’s constant has units of:
a) J·m
b) J·Hz
c) J·s
d) N·s
