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Electromagnetic spectrum: NEET MCQs with answers

51 questions on this topic of Electromagnetic Waves (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. An amateur radio operator transmits a signal at a wavelength of 15 m in free space. What is the operational frequency of this transmission?

  1. 2.0 MHz
  2. 2.0 GHz
  3. 200 MHz
  4. 20 MHz
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Answer: (D) 20 MHz

Using the relation c = νλ, we have ν = c / λ = (3.0 × 10⁸ m/s) / (15 m) = 0.2 × 10⁸ Hz = 20 × 10⁶ Hz = 20 MHz.

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2. In a diagnostic medical procedure, radiation of wavelength 0.15 nm is used to inspect bone structures. The photon energy associated with this radiation is approximately: (Take Planck constant h = 6.63 × 10⁻³⁴ J·s and 1 eV = 1.6 × 10⁻¹⁹ J)

  1. 8.3 keV
  2. 1.2 keV
  3. 15.5 keV
  4. 0.83 keV
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Answer: (A) 8.3 keV

Energy E = hc / λ. Using hc ≈ 1242 eV·nm, E = (1242 eV·nm) / (0.15 nm) = 8280 eV ≈ 8.3 keV. Alternatively, E = (6.63 × 10⁻³⁴ × 3 × 10⁸) / (0.15 × 10⁻⁹ × 1.6 × 10⁻¹⁹) J/eV ≈ 8287 eV ≈ 8.3 keV.

hard

3. An electromagnetic wave of frequency 4.0 × 10¹⁴ Hz passes from vacuum into a non-magnetic dielectric medium of dielectric constant K = 4.0. The wavelength of this wave in the medium is:

  1. 375 nm
  2. 750 nm
  3. 187.5 nm
  4. 500 nm
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Answer: (A) 375 nm

In vacuum, wavelength λ₀ = c / ν = (3.0 × 10⁸ m/s) / (4.0 × 10¹⁴ Hz) = 7.5 × 10⁻⁷ m = 750 nm. The refractive index of a non-magnetic medium is n = √(εᵣ μᵣ) = √(4.0 × 1) = 2.0. Therefore, the wavelength inside the medium is λ = λ₀ / n = 750 nm / 2.0 = 375 nm.

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4. Which of the following statements is INCORRECT regarding electromagnetic waves in the visible spectrum?

  1. Red light has a longer wavelength than violet light.
  2. Violet light photons carry more energy than red light photons.
  3. The speed of red light in vacuum is greater than the speed of violet light in vacuum.
  4. The visible spectrum spans roughly from 400 nm to 700 nm.
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Answer: (C) The speed of red light in vacuum is greater than the speed of violet light in vacuum.

All electromagnetic waves, regardless of their wavelength or frequency, propagate at the exact same constant speed c (3 × 10⁸ m/s) in vacuum. Therefore, red light and violet light have identical speeds in vacuum.

easy

5. An electromagnetic wave has a frequency of 1 MHz in vacuum. Its wavelength is:

  1. 30 m
  2. 0.00333 m
  3. 3000 m
  4. 300 m
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Answer: (D) 300 m

All EM waves travel at c in vacuum, so λ = c/f = 3 × 10⁸/(1 × 10⁶) = 300 m.

hard

6. A monochromatic sodium vapor lamp emits light of wavelength 589 nm with a total radiant power output of 50 W. Assuming the emission is uniform in all directions, calculate the rate of emission of photons per second from the lamp: (Take h = 6.63 × 10⁻³⁴ J·s)

  1. 7.2 × 10¹⁸ photons/s
  2. 2.5 × 10¹⁹ photons/s
  3. 4.8 × 10²¹ photons/s
  4. 1.5 × 10²⁰ photons/s
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Answer: (D) 1.5 × 10²⁰ photons/s

Energy per photon is E = hc / λ = (6.63 × 10⁻³⁴ × 3 × 10⁸) / (589 × 10⁻⁹) ≈ 3.377 × 10⁻¹⁹ J. The rate of photon emission is N = P / E = 50 / (3.377 × 10⁻¹⁹) ≈ 1.48 × 10²⁰ photons/s ≈ 1.5 × 10²⁰ photons/s.

easy

7. An electromagnetic wave has a frequency of 3000 MHz in vacuum. Its wavelength is:

  1. 1 m
  2. 10 m
  3. 0.01 m
  4. 0.1 m
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Answer: (D) 0.1 m

All EM waves travel at c in vacuum, so λ = c/f = 3 × 10⁸/(3000 × 10⁶) = 0.1 m.

easy

8. An electromagnetic wave has a frequency of 100 MHz in vacuum. Its wavelength is:

  1. 0.3 m
  2. 30 m
  3. 0.333 m
  4. 3 m
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Answer: (D) 3 m

All EM waves travel at c in vacuum, so λ = c/f = 3 × 10⁸/(100 × 10⁶) = 3 m.

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9. Match List-I (Electromagnetic Wave) with List-II (Typical Application): List-I: P. Microwaves Q. Ultraviolet rays R. Infrared rays S. Gamma rays List-II: 1. Destroying cancerous cells in medicine 2. RADAR systems for aircraft navigation 3. Water purifiers to kill germs 4. Night-vision cameras and haze photography

  1. P-2, Q-4, R-3, S-1
  2. P-3, Q-2, R-4, S-1
  3. P-2, Q-3, R-4, S-1
  4. P-1, Q-3, R-4, S-2
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Answer: (C) P-2, Q-3, R-4, S-1

Microwaves are used in RADAR systems due to their short wavelengths; ultraviolet light kills bacteria and germs in purifiers; infrared is used in night-vision and thermal imaging; and gamma rays are used in radiotherapy to destroy cancerous tissues.

hard

10. Consider the following frequencies corresponding to four different electromagnetic waves: f₁ = 5.0 × 10¹⁹ Hz, f₂ = 6.0 × 10¹⁴ Hz, f₃ = 2.0 × 10¹⁰ Hz, f₄ = 8.0 × 10⁶ Hz. The regions to which these frequencies belong are respectively:

  1. Gamma ray, Ultraviolet ray, Infrared ray, Radio wave
  2. X-ray, Infrared ray, Radio wave, Microwave
  3. Gamma ray, Visible light, Microwave, Radio wave
  4. X-ray, Visible light, Microwave, Short radio wave
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Answer: (C) Gamma ray, Visible light, Microwave, Radio wave

f₁ = 5.0 × 10¹⁹ Hz lies in the gamma-ray domain (> 3 × 10¹⁹ Hz). f₂ = 6.0 × 10¹⁴ Hz lies in the visible spectrum (4 × 10¹⁴ Hz to 7.5 × 10¹⁴ Hz). f₃ = 2.0 × 10¹⁰ Hz (20 GHz) belongs to microwaves (1 GHz to 300 GHz). f₄ = 8.0 × 10⁶ Hz (8 MHz) belongs to radio waves.

easy

11. Which segment of the electromagnetic spectrum is primarily used in LASIK eye surgery for corneal reshaping?

  1. Infrared rays
  2. Hard X-rays
  3. Microwaves
  4. Ultraviolet rays
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Answer: (D) Ultraviolet rays

Excimer lasers used in LASIK eye surgery emit narrow, high-precision beams of ultraviolet radiation. Because of their short wavelength and high photon energy, UV lasers can precisely ablate thin layers of corneal tissue without causing thermal damage to surrounding tissue.

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12. Identify the electromagnetic radiation whose typical wavelength is around 1 mm to 0.1 m and is extensively utilized in aircraft navigation radar systems.

  1. Radio waves
  2. Microwaves
  3. Infrared waves
  4. Ultraviolet rays
Show answer and explanation

Answer: (B) Microwaves

Microwaves have wavelengths in the range of roughly 0.1 m to 1 mm (frequencies 3 GHz to 300 GHz). Due to their relatively short wavelengths compared to standard radio waves, they can be formed into narrow directional beams, making them ideal for radar systems in aircraft navigation.

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