# Physics Questions and Answers – Electromagnetic Spectrum

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This set of Physics Multiple Choice Questions & Answers (MCQs) focuses on “Electromagnetic Spectrum”.

1. On what basis is the classification of electromagnetic waves done?
a) Electromagnetic spectrum
b) Electric field
c) Magnetic field
d) Propagation constant

Explanation: The classification of electromagnetic waves done according to the frequency called the electromagnetic spectrum. The basic difference various type of electromagnetic waves lies in their wavelength or frequency since all of them travel through vacuum at the same speed and also, the waves differ in their mode of interaction with matter.

2. Which among the following has a frequency range of 500 kHz to 1000 MHz?
a) Microwaves
b) Infrared Waves
d) Gamma Rays

Explanation: Radio Waves are generally in the frequency range ➔ 500 kHz to 1000 MHz. Radio waves are used for long-distance communication, such as in television, mobiles, and radios. These devices receive radio waves and convert them to mechanical vibrations in the speaker to create sound waves.

3. What is the full form of UHF?
a) Under heavy frequency
b) Ultra-high frequency
c) Ultra heavy frequency
d) Under high frequency

Explanation: UHF stands for Ultra high frequency. Cellular phones used radio waves to transmit voice communication in the Ultra high-frequency Band. The UHF band extends from 900 MHz to 5 x 109 Hz or 5000 MHz. Radio waves are produced by oscillating circuits having an inductor and capacitor.

4. Microwaves are produced by klystrons.
a) True
b) False

Explanation: Yes, microwaves are produced by special vacuum tubes such as klystrons and magnetrons. Microwaves are high-frequency radio waves of frequency range 1 G Hz to 300 G Hz. Microwaves are used in radar systems and the study of atomic and molecular structure.

5. Which among the following is an application of microwaves?
a) Solar cells
b) Ovens
d) Burglar alarms

Explanation: Microwave ovens are an application of microwaves. In microwave ovens, the frequency of microwave produced is matched with the natural frequency of water molecules so that resonance occurs and water molecules in the material vibrate at the higher amplitude and transfer energy to nearby food molecules, results in heating the food.

6. Identify the electromagnetic wave which is also known as heatwaves.
b) Gamma Waves
c) X-Rays
d) Infrared Waves

Explanation: Infrared waves are heat radiations also known as heatwaves. These waves are produced by hot bodies and molecules. They do heating because water molecules present in most of the materials readily absorb infrared waves and their thermal motion increases, so they heat themselves and also heat their surroundings.

7. Pick out the electromagnetic wave which is highly harmful to humans.
b) Ultraviolet Rays
c) Microwaves
d) Infrared Waves

Explanation: Ultraviolet (UV) radiations in large quantities are highly harmful to humans. These rays in solar radiation on reaching earth are absorbed by the ozone layer in the atmosphere. UV rays are produced by special lamps such as mercury and from arc lamps and by very hot bodies like the sun.

8. Stephen is thinking of going to the hospital for checking whether he has tuberculosis since he believes that he may have some early symptoms of the disease. His friend suggests him to do an x-ray checkup to clear up the confusion. Is the suggestion made by Stephen’s friend going to help him?
a) True
b) False

Explanation: No, the suggestion is not going to help Stephen. Even though X-rays have higher energy than light waves and can pass through the body, it is not of great value in the early diagnosis of pathological conditions of bones and joints like tuberculosis. This is because, in the early stages of the disease, the damage to the lungs may not yet have become sufficiently marked in the X-ray to be detectable, so people who have active tuberculosis can be missed.

9. Monica wanted to take photographs of a monument. But since the surrounding is filled with smoke, she is not able to take good photos. Which one of the following electromagnetic waves can be used in this situation to help Monica?
a) X-rays
b) Gamma rays
c) Ultraviolet rays
d) Infrared waves

Explanation: Infrared waves can be used to take photographs during conditions of smoke, fog, etc. as these waves are scattered less than visible rays and hence travel longer distances through the atmosphere. So, using infrared waves can help Monica out.

10. Which is the frequency range of gamma rays from the following?
a) 1 × 1018 to 3 × 1022 Hz
b) 3 × 10-18 to 5 × 1022 Hz
c) 3 × 1018 to 5 × 1022 Hz
d) 3 × 10-18 to 5 × 10-22 Hz

Explanation: The frequency range of Gamma rays is 3 × 1018 to 5 × 1022 Hz. These rays have a wavelength of 6 × 10-13 to 10-10m. Gamma rays are produced in nuclear reactions and are also emitted by radioactive nuclei.

11. A radio can tune to any station in the frequency range from 2.5 MHz to 15 MHz bands. What is the corresponding wavelength band?
a) 30 m to 130 m
b) 40 m to 140 m
c) 20 m to 120 m
d) 10 m to 110 m

Explanation: Maximum wavelength in the band will correspond to the lowest frequency:
c=fminλmax
λmax=$$\frac {c}{f_{min}}=\frac {3\times 10^8}{2.5\times 10^6}$$=120 m
Minimum wavelength in the band will correspond to the highest frequency:
c=fmaxλmin
λmin=$$\frac {c}{f_{max}} =\frac {3\times 10^8}{15 \times 10^6 }$$=20 m
Therefore, the corresponding wavelength band is 20 m to 120 m.

12. The speeds of microwaves, infrared waves, and ultraviolet waves are Vm, Vi, and Vu respectively. Identify the correct combination showing the different waves in vacuum.
a) Vm > Vi > Vu
b) Vm = Vi = Vu
c) Vm < Vi < Vu
d) Vm > Vi < Vu

Explanation: The correct combination is ➔ Vm = Vi = Vu. This is because, in vacuum, all the electromagnetic waves in question will travel at the same speed. The speed with which they travel in vacuum is the speed of light. (c = 3 × 108 m/s).

13. Electromagnetic radiation has an energy of 15 keV. Then this radiation belongs to which region of electromagnetic waves?
a) X rays
b) Infrared rays
c) Gamma rays
d) UV rays

Explanation: Given: Energy (E) = 15 keV
The required equation ➔ λ(in Å)=$$\frac {hc}{E(in \, eV)}$$
λ=$$\frac {12400}{15 \times 10^3}$$
λ=0.826 Å=0.826 × 10-10 m
X rays cover wavelengths ranging from above 10-8 m to 10-13 m. Therefore, this electromagnetic radiation belongs to the region of X rays.

14. Pick out the correct increasing order of energy of electromagnetic waves from the following.
a) Einfrared < Emicro < Evisible < Eultraviolet < Egamma
b) Emicro < Einfrared < Evisible < Eultraviolet < Egamma
c) Emicro < Einfrared < Evisible < Egamma < Eultraviolet
d) Emicro < Einfrared < Eultraviolet < Evisible < Egamma

Explanation: The energy of electromagnetic waves is directly proportional to the frequency of the electromagnetic waves. So the order of frequency is given as:
vmicro < vinfrared < vvisible < vultraviolet < vgamma
Since E=hv ➔ E ∝ v
The order of energy is as follows:
Emicro < Einfrared < Evisible < Eultraviolet < Egamma

15. Which of the following cannot travel in vacuum?
b) Gamma Waves
c) Infrared Waves
d) Infrasonic waves

Explanation: Radio waves, gamma waves, and infrared waves are electromagnetic waves and due to this they do not need any material medium to travel and hence, can travel in vacuum. Whereas, infrasonic waves are mechanical waves and so, they need a material medium to travel. Therefore, infrasonic waves cannot travel in vacuum.

Sanfoundry Global Education & Learning Series – Physics – Class 12.

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