# Physics Questions and Answers – Electromagnetic Waves

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

1. What is the ratio of the speed of infrared and ultraviolet rays in a vacuum?
a) 1:5
b) 2:1
c) 1
d) 0

Explanation: Ratio = 1 because the speed of an electromagnetic wave in vacuum is independent of its wavelength or frequency. Therefore, the ratio of speed of infrared and ultraviolet rays in a vacuum is one.

2. What is the velocity of electromagnetic wave in free space?
a) c = √μ0ε0
b) c = $$\frac {1}{\sqrt {\mu_0 \varepsilon_0}}$$
c) c = $$\frac {1}{\mu_0 \varepsilon_0}$$
d) c = μ0ε0

Explanation: The value of c is the same for all types of electromagnetic waves because the constants μ0 and ε0 do not depend on the frequency or wavelength of the electromagnetic waves. So, the velocity of electromagnetic wave in free space is given as:
c = $$\frac {1}{\sqrt {\mu_0 \varepsilon_0}}$$.

3. Which basic oscillatory circuit is used these days to produce electromagnetic waves?
a) LC oscillatory circuit
b) LCR circuit
c) Inductive circuit
d) Capacitive circuit

Explanation: LC-oscillatory circuit is used these days to produce electromagnetic waves. It generates electromagnetic waves of frequency, v = $$\frac {1}{2 \pi \sqrt {LC}}$$. Therefore, the basic oscillatory circuit is used these days to produce electromagnetic waves is LC oscillatory circuit.
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4. Is the ratio of frequencies of UV rays and IR rays in the glass more than, less than or equal to 1?
a) Insufficient data
b) Equal to 1
c) Less than 1
d) More than 1

Explanation: The ratio of frequencies of UV rays and IR rays in the glass is more than 1.This is because the frequency of UV rays is greater than that of infrared rays. This situation is applicable in glass or vacuum or air.

5. For which frequency of light, the human eye is most sensitive?
a) 55.405 × 1014 Hz
b) 95.405 × 1014 Hz
c) 5.405 × 1014 Hz
d) 79.405 × 1014 Hz

Explanation: Human eye is sensitive to light of wavelength ➔ λ =5550 angstrom.
So its frequency is v = $$\frac {c}{\lambda }$$
v = 3 × $$\frac {10^8}{5550}$$ × 10-10
v = 5.405 × 1014 Hz

6. Microwaves can’t be used for long-distance transmission of signals.
a) True
b) False

Explanation: Due to their smaller wavelength, microwaves are not bent by objects of normal dimensions. So they can be used to beam signal in a particular direction. Therefore, microwaves can be used for long-distance transmission of signals.

7. If the wavelength of electromagnetic radiation is doubled, what will happen to the energy of photons?
a) Doubled
b) Halved
c) Remains the same
d) Becomes zero

Explanation: Energy of a photon,
E = hv = $$\frac {hc}{\lambda }$$.
E ∝ $$\frac {1}{\lambda }$$.
When the wavelength of electromagnetic radiation is doubled, the energy of the photons is halved.

8. What is the time period of visible light for which human eye is most sensitive?
a) 1.85 × 1015 s
b) 1.85 × 10-15 s
c) 3 × 108s
d) 5 × 1016 s

Explanation: T = $$\frac {1}{v}$$
T = $$\frac {\lambda }{c}$$
T = 5550 × $$\frac {10^{-10}}{3}$$ × 108
T = 1.85 × 10-15 s.

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