Velocity of Light MCQs

Question 1

Frequency of a radio wave is 15 MHz. Its wavelength is:

Accepted Answer

20 m

Answer

30 m

Answer

40 m

Answer

15 m

Question 2

Which of the following is not a type of electromagnetic radiation?

Accepted Answer

Sound waves

Answer

Radio waves

Answer

X-rays

Answer

Ultraviolet radiation

Question 3

The range of wavelength of UV rays is:

Accepted Answer

100–400 nm

Answer

400–800 nm

Answer

200–600 nm

Answer

500–900 nm

Question 4

Which classical method was used to measure the speed of light with high accuracy?

Accepted Answer

Michelson's rotating-mirror method

Answer

Fizeau's toothed-wheel method

Answer

Foucault's rotating-mirror method

Answer

None of these

Question 5

The speed of electromagnetic waves in vacuum is:

Accepted Answer

$\frac{1}{\sqrt{\mu_0\varepsilon_0}}$

Answer

$\sqrt{\frac{\mu_0}{\varepsilon_0}}$

Answer

$\frac{1}{\mu_0\varepsilon_0}$

Answer

$\mu_0\varepsilon_0$

Question 6

Time taken by light to cross a glass slab of thickness $5\ \mathrm{cm}$ and refractive index $1.5$ is:

Accepted Answer

$2.5\times10^{-10}\ \mathrm{s}$

Answer

$2\times10^{-10}\ \mathrm{s}$

Answer

$3\times10^{-10}\ \mathrm{s}$

Answer

$3.5\times10^{-10}\ \mathrm{s}$

Question 7

For which colour is the speed of light maximum in diamond?

Accepted Answer

Red

Answer

Yellow

Answer

Blue

Answer

Violet

Question 8

The refractive index of diamond is $2.4$. What is the speed of light in diamond?

Accepted Answer

$1.25\times10^8\ \mathrm{m/s}$

Answer

$2.5\times10^8\ \mathrm{m/s}$

Answer

$1.5\times10^8\ \mathrm{m/s}$

Answer

$2.0\times10^8\ \mathrm{m/s}$

Question 9

Which sequence of electromagnetic waves is in decreasing order of frequency?

Accepted Answer

X-rays, visible light, microwaves

Answer

Infrared, visible light, ultraviolet

Answer

X-rays, visible light, ultraviolet

Answer

Microwaves, ultraviolet, X-rays

Question 10

The speed of light in a liquid is $2\times10^8\ \mathrm{m/s}$. What is its refractive index?

Accepted Answer

$1.5$

Answer

$1.2$

Answer

$0.64$

Answer

$0.8$

Question 11

Which sequence gives electromagnetic waves in order of increasing wavelength?

Accepted Answer

Ultraviolet, visible, infrared, microwaves

Answer

Gamma rays, infrared, visible, radio

Answer

Microwaves, ultraviolet, visible, infrared

Answer

Radio, microwaves, infrared, visible

Question 12

Which radiation has a higher frequency than infrared and a shorter wavelength than X-rays?

Accepted Answer

Gamma rays

Answer

Microwaves

Answer

Ultraviolet

Answer

Radio waves

Question 13

An electromagnetic wave in vacuum has frequency $100\ \mathrm{Hz}$. Its wavelength is:

Accepted Answer

$3\times10^6\ \mathrm{m}$

Answer

$2\times10^6\ \mathrm{m}$

Answer

$2\times10^8\ \mathrm{m}$

Answer

$3\times10^8\ \mathrm{m}$

Question 14

Which electromagnetic radiation has lower photon energy than infrared?

Accepted Answer

Microwaves

Answer

Visible light

Answer

X-rays

Answer

None of these

Question 15

The speed of light in vacuum depends on:

Accepted Answer

None of these

Answer

Frequency

Answer

Wavelength

Answer

Amplitude

Question 16

In a Fizeau toothed-wheel experiment, the wheel has $720$ teeth, the reflector is $9.633\ \mathrm{km}$ away, and the first extinction is observed at $750\ \mathrm{rpm}$. Calculate the speed of light.

Accepted Answer

$3.47\times10^8\ \mathrm{m/s}$

Answer

$2.10\times10^8\ \mathrm{m/s}$

Answer

$4.10\times10^8\ \mathrm{m/s}$

Answer

$2.90\times10^8\ \mathrm{m/s}$

Question 17

In a rotating-mirror experiment, a beam travels to a reflector $20\ \mathrm{m}$ away and back. If the mirror rotates at $20\ \mathrm{rev/s}$, what angle does it turn during the round trip?

Accepted Answer

$1.68\times10^{-5}\ \mathrm{rad}$

Answer

$5.33\times10^{-4}\ \mathrm{rad}$

Answer

$6.22\times10^{-4}\ \mathrm{rad}$

Answer

$3.33\times10^{-4}\ \mathrm{rad}$

Question 18

In Michelson's rotating-octagonal-mirror experiment, if the distance between mirrors is doubled, the required rotational speed for the same image condition becomes:

Accepted Answer

Half

Answer

Unchanged

Answer

Twice

Answer

Four times

Medium / Condition	Velocity / Formula
Vacuum / free space	\(c=3\times10^8\ m/s\)
Vacuum formula	\(c=\frac{1}{\sqrt{\epsilon_0\mu_0}}\)
Electromagnetic wave relation	\(c=\frac{E}{B}=3\times10^8\ m/s\)
Medium
Medium	\(v=\frac{c}{\sqrt{\epsilon_r\mu_r}}\)
Using refractive index	\(v=\frac{c}{\mu}\)

Medium	Velocity of light	Examples
Isotropic optical medium	Same in all directions	Water, air, glass
Anisotropic optical medium	Different in different directions	Crystals like calcite, quartz

Condition	Formula
First eclipse	\(c=4mnd\)
\(p^{th}\) eclipse	\(c=\frac{4mnd}{2p-1}\)

Symbol	Meaning
\(m\)	Number of teeth of rotating wheel
\(n\)	Number of rotations per second of wheel when first eclipse occurs
\(d\)	Distance between wheel and mirror
\(p\)	Number of eclipses

Formula	Expression
Velocity of light	\(c=\frac{8\pi NR^2a}{(b+R)x}\)
Using mirror rotation angle	\(c=\frac{4\pi NR}{\theta}\)

31Velocity of Light

Table 1: Velocity of Light

Table 1: Isotropic vs Anisotropic Optical Medium

Table 1: Fizeau's Method Formulae

Table 2: Symbols

Table 1: Foucault's Method Formulae

Table 2: Symbols

Table 1: Symbols

Table 1: Colour, Velocity and Sensitivity

Table 1: Changes During Refraction

Table 1: Important Points

Table 1: Velocity of Light One-Liners

More CEE preparation resources

Symbol	Meaning
\(R\)	Distance between plane mirror and concave mirror
\(N\)	Number of rotations of rotating mirror per second
\(a\)	Distance of object from achromatic lens
\(b+R\)	Distance of image from achromatic lens
\(x\)	Shift of image
\(\theta\)	Angle turned by mirror

Symbol	Meaning
\(n\)	Number of rotations per second of octagonal mirror
\(d\)	Distance travelled by light

Fact	Answer
Colour of light depends on	Wavelength
All colours in vacuum	Same speed
In medium	Red has maximum speed
In medium	Violet has minimum speed
Photographic plate most sensitive to	Violet light
Photographic plate least sensitive to	Red light
Reason	Red has larger wavelength

Fact	Answer
Objects visible from all directions	Due to scattering of light
Colour of light	Depends on wavelength
On refraction	Velocity and wavelength change; frequency remains same
Speed of colours in vacuum	All colours have same speed
Speed of colours in medium	Red maximum, violet minimum
Photographic plate sensitivity	Most sensitive to violet, least to red
Light slower in water than air confirmed by	Foucault's method
Most accurate method	Michelson's method
Rectilinear propagation	Light appears to travel in straight line because wavelength is very small
Ray optics valid when	Characteristic dimensions are much larger than wavelength of light

Fact	Answer
Velocity of light in vacuum	\(3\times10^8\ m/s\)
Vacuum speed formula	\(c=\frac{1}{\sqrt{\epsilon_0\mu_0}}\)
EM wave relation	\(c=\frac{E}{B}\)
Speed in medium	\(v=\frac{c}{\mu}\)
Higher refractive index	Lower velocity of light
Isotropic medium	Velocity same in all directions
Anisotropic medium	Velocity different in different directions
Fizeau's method	Rotating toothed wheel
Fizeau first eclipse	\(c=4mnd\)
Fizeau \(p^{th}\) eclipse	\(c=\frac{4mnd}{2p-1}\)
Foucault's method	Rotating mirror
Foucault evidence	Supports Huygens' wave theory
Foucault confirmed	Light slower in water than air
Michelson's method	Octagonal mirror
Michelson formula	\(c=8nd\)
Most accurate method	Michelson's method
During refraction	Velocity and wavelength change
During refraction	Frequency unchanged
Colour depends on	Wavelength
In vacuum colour speeds	All same
In medium fastest colour	Red
In medium slowest colour	Violet
Photographic plate most sensitive	Violet
Light travels straight because	Wavelength is very small
Ray optics condition	Object/aperture dimensions ≫ wavelength