Electromagnetic Spectrum and Applications – Electromagnetic spectrum (radio waves: NEET MCQs with Answers

15 free questions on Electromagnetic Spectrum and Applications – Electromagnetic spectrum (radio waves for NEET (UG), starting with previous-year NEET and AIPMT questions. Try each one, then tap Show answer for the answer and a short explanation.

Syllabus covered: Electromagnetic spectrum (radio waves, microwaves, infrared, visible, ultraviolet. X-rays. Gamma rays), Applications of e.m. waves.

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Question 1

Previous-year question
Which colour of the light has the longest wavelength? [NEET 2019]
  1. red
  2. blue
  3. green
  4. violet
Show answer
Answer: (A) red
Explanation: Across the visible spectrum the wavelength increases from violet (about 400 nm) through blue and green to red (about 700 nm). Red therefore has the longest wavelength and the lowest frequency.

Question 2

Previous-year question
The energy of the em waves is of the order of 15 keV. To which part of the spectrum does it belong? [AIPMT 2015]
  1. Infra-red rays
  2. Ultraviolet rays
  3. γ-rays
  4. X-rays
Show answer
Answer: (D) X-rays
Explanation: An energy of 15 keV corresponds to a wavelength of about 0.83 Å. Wavelengths of this order define the X-ray band; γ-rays would require energies typically in the MeV range.

Question 3

Previous-year question
Which of the following is the longest wave? [AIPMT 1989]
  1. X-rays
  2. γ-rays
  3. Microwaves
  4. Radiowaves
Show answer
Answer: (D) Radiowaves
Explanation: Typical wavelength ranges are: γ-rays 6 × 10⁻¹⁴–10⁻¹⁰ m, X-rays 10⁻¹³–3 × 10⁻⁸ m, microwaves 10⁻³–0.3 m, and radio waves greater than 0.1 m. Radio waves are therefore the longest.

Question 4

Previous-year question
Pick out the longest wavelength from the following types of radiations. [AIPMT 1990]
  1. Blue light
  2. Gamma rays
  3. X-rays
  4. Red light
Show answer
Answer: (D) Red light
Explanation: Gamma rays (6 × 10⁻¹⁴ to 10⁻¹⁰ m) and X-rays (10⁻¹³ to 3 × 10⁻⁸ m) are far shorter than visible light. Within the visible band, which runs from about 4000 Å to 7800 Å, red lies at the long-wavelength end and blue near the short end. Red light therefore has the longest wavelength here.

Question 5

Previous-year question
The frequency of an electromagnetic wave which is best suited to observe a particle of radius 3 × 10⁻⁴ cm is of the order of [AIPMT 1991]
  1. 10¹⁵
  2. 10¹⁴
  3. 10¹³
  4. 10¹²
Show answer
Answer: (B) 10¹⁴
Explanation: To resolve an object, the wavelength used must be comparable to its size, so take λ ≈ 3 × 10⁻⁴ cm. Then ν = c/λ = (3 × 10¹⁰ cm/s)/(3 × 10⁻⁴ cm) = 10¹⁴ Hz.

Question 6

Previous-year question
The structure of solids is investigated by using [AIPMT 1992]
  1. cosmic rays
  2. X-rays
  3. γ-rays
  4. infrared radiations
Show answer
Answer: (B) X-rays
Explanation: X-ray wavelengths are comparable to interatomic spacings in crystals (a few ångströms), so crystals act as natural diffraction gratings for them. Techniques such as the Laue spot method and the rotating-crystal method exploit this to reveal the structure of solids.

Question 7

Previous-year question
The ozone layer blocks radiations of wavelength [AIPMT 1999]
  1. less than 3 × 10⁻⁷ m
  2. equal to 3 × 10⁻⁷ m
  3. more than 3 × 10⁻⁷ m
  4. all of the above
Show answer
Answer: (A) less than 3 × 10⁻⁷ m
Explanation: The ozone layer, lying roughly 30–50 km above the earth's surface, absorbs the harmful ultraviolet component of sunlight. Ultraviolet radiation spans about 100 Å to 4000 Å, so the layer blocks wavelengths shorter than about 3 × 10⁻⁷ m (3000 Å).
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Question 8

Previous-year question
The frequencies of γ-rays, X-rays and ultraviolet rays are a, b and c respectively. Then [AIPMT 2000]
  1. a > b > c
  2. a < b < c
  3. a = b = c
  4. a > c > b
Show answer
Answer: (A) a > b > c
Explanation: γ-rays have frequencies of about 3 × 10¹⁸–3 × 10²¹ Hz, X-rays about 3 × 10¹⁶–3 × 10¹⁸ Hz, and ultraviolet about 7.5 × 10¹⁴–3 × 10¹⁶ Hz. Hence a > b > c — the reverse of their wavelength order.

Question 9

Previous-year question
What is the cause of the greenhouse effect? [AIPMT 2002]
  1. Infrared rays
  2. Ultraviolet rays
  3. X-rays
  4. Radio-waves
Show answer
Answer: (A) Infrared rays
Explanation: Glass and atmospheric greenhouse gases transmit short-wavelength visible light but absorb the longer-wavelength infrared radiation re-emitted by the warmed surfaces inside. The infrared energy is thus trapped, and the enclosure — or the planet — stays warm.

Question 10

Previous-year question
Which of the following has the minimum wavelength? [AIPMT 2002]
  1. X-rays
  2. Ultraviolet rays
  3. γ-rays
  4. Cosmic rays
Show answer
Answer: (D) Cosmic rays
Explanation: The wavelength falls as the energy of the radiation rises, so the order is ultraviolet > X-rays > γ-rays. Cosmic radiation lies beyond γ-rays in energy, so it is taken to have the smallest wavelength of the four, which is the official AIPMT answer.

Question 11

Previous-year question
Which of the following rays are NOT electromagnetic waves? [AIPMT 2003]
  1. β-rays
  2. Heat rays
  3. X-rays
  4. γ-rays
Show answer
Answer: (A) β-rays
Explanation: β-rays are streams of fast-moving electrons — particles with rest mass and charge — and are therefore not electromagnetic radiation. Heat (infrared) rays, X-rays and γ-rays are all electromagnetic waves and can travel through vacuum.

Question 12

Previous-year question
If λ_v, λ_x and λ_m represent the wavelengths of visible light, X-rays and microwaves respectively, then [AIPMT 2005]
  1. λ_m > λ_x > λ_v
  2. λ_v > λ_m > λ_x
  3. λ_m > λ_v > λ_x
  4. λ_v > λ_x > λ_m
Show answer
Answer: (C) λ_m > λ_v > λ_x
Explanation: Typical wavelength ranges are: microwaves 10⁻³–10⁻¹ m, visible light 4000–7000 Å (≈10⁻⁷ m), and X-rays 10⁻¹¹–10⁻⁸ m. Hence λ_m > λ_v > λ_x.

Question 13

Previous-year question
The decreasing order of wavelength of infrared, microwave, ultraviolet and gamma rays is [AIPMT 2011]
  1. gamma rays, ultraviolet, infrared, microwaves
  2. microwaves, gamma rays, infrared, ultraviolet
  3. infrared, microwave, ultraviolet, gamma rays
  4. microwave, infrared, ultraviolet, gamma rays
Show answer
Answer: (D) microwave, infrared, ultraviolet, gamma rays
Explanation: In the electromagnetic spectrum the wavelength decreases in the sequence radio → microwave → infrared → visible → ultraviolet → X-ray → gamma. For the four listed, the decreasing order is therefore microwave > infrared > ultraviolet > gamma rays.

Question 14

Previous-year question
The condition under which a microwave oven heats up a food item containing water molecules most efficiently is [NEET 2013]
  1. the frequency of the microwave must match the resonant frequency of the water molecules
  2. the frequency of the microwave has no relation with the natural frequency of water molecules
  3. microwaves are heat waves, so they always produce heating
  4. infra-red waves produce heating in a microwave oven
Show answer
Answer: (A) the frequency of the microwave must match the resonant frequency of the water molecules
Explanation: Microwave ovens work by resonant absorption. When the microwave frequency matches the natural rotational frequency of the water molecules, the molecules are set into vigorous oscillation and the energy absorbed is greatest; this energy is then shared with the surrounding food as heat.

Question 15

Previous-year question
The energy of EM waves is of the order of 15 keV. To which part of the spectrum does it belong? [AIPMT 2015]
  1. X-rays
  2. Infrared rays
  3. Ultraviolet rays
  4. γ-rays
Show answer
Answer: (A) X-rays
Explanation: From E = hc/λ, λ = hc/E = (6.624 × 10⁻³⁴ × 3 × 10⁸)/(15 × 10³ × 1.6 × 10⁻¹⁹) ≈ 0.83 × 10⁻¹⁰ m = 0.83 Å. Wavelengths of the order of an ångström fall squarely in the X-ray region.

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