Physics

Electromagnetic Waves and Spectrum

659 Questions

Electromagnetic waves and spectrum questions test a candidate's understanding of radiation frequencies, wavelengths, and the properties of different rays like infrared, ultraviolet, and visible light. Concepts also cover practical applications in astronomy and the fundamental speed of light calculations. This physics topic appears regularly in general science sections of major competitive examinations.

UV rays propertiesElectromagnetic radiation frequencyWavelength identificationSpeed of light calculationsBlack body radiation

Electromagnetic Waves and Spectrum Questions

Multiple choice physics option a: relativity the nature of light speed of light and optical density introduction to light

If $v _s$ , $v _x$ and $v _m$ are the velocities of soft gamma rays, X-rays and Microwaves respectively in vacuum, then

  1. $v _s < v _x < v _m$
  2. $v _s = v _x = v _m$
  3. $v _s > v _x > v _m$
  4. $v _x < v _s < v _m$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Soft gamma rays, $x-$rays and microwaves are namely eletromagnetic having different wavelengths. 

But, all of them propagate through space with the same speed,
$e=3\times 10^{8}\ m/s$
 so,
$v _{s}=v _{x}=v _{m}$

Multiple choice physics option a: relativity the nature of light speed of light and optical density introduction to light

Three observers $A,B$ and $C$ measure the speed of light coming from a source to be $v _A,v _B$ and $v _C$. The observer $A$ moves towards the source and $C$ moves away from the source at the same speed. The observer $B$ stays stationary. The surrounding space is vacuum everywhere.

  1. $\displaystyle v _A>v _B>v _C$
  2. $\displaystyle v _A < v _B < v _C$
  3. $\displaystyle v _A=v _B=v _C$
  4. $\displaystyle v _B=\frac{1}{2}(v _A+v _C)$
Reveal answer Fill a bubble to check yourself
C,D Correct answer
Explanation

Relativity states that the speed of light in vacuum will be same independent of the frame of reference. The frame of reference can be stationary or moving.
So all  three will find the speed of light to be $ c $
Also $ c=\dfrac{1}{2}(c+c) $
So, (C) and (D) are correct.

Multiple choice physics option a: relativity the nature of light speed of light and optical density introduction to light

An electromagnetic wave of frequency $v=3.0:MHz$ passes from vacuum into a dielectric medium with permittivity $\epsilon=4.0$. Then

  1. wavelength is halved and frequency remains unchanged.

  2. wavelength is doubled and frequency becomes half.

  3. wavelength is doubled and frequency remains unchanged.

  4. wavelength and frequency both remain unchanged.
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Frequency remains constant during refraction.

$\displaystyle v _{med}=\dfrac{1}{\sqrt{\mu _0\epsilon _0\times4}}=\dfrac{c}{2}$

$\displaystyle\dfrac{\lambda _{med}}{\lambda _{air}}=\dfrac{v _{med}}{v _{air}}=\dfrac{\displaystyle\dfrac{c}{2}}{c}=\dfrac{1}{2}$

$\therefore$ wavelength is halved and frequency remains unchanged.

Multiple choice physics option a: relativity the nature of light speed of light and optical density introduction to light

The electromagnetic waves

  1. travel with the the speed of sound.

  2. travel with the the same speed in all media.

  3. travel in free space with the speed of light.

  4. do not travel through a medium.
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The electromagnetic waves of all wavelengths travel with the same speed in space which is equal to velocity of light.

Multiple choice physics option a: relativity the nature of light speed of light and optical density introduction to light

The apparent wavelength of light from a star moving away from the earth is 0.02% more than actual wavelength. What is the velocity of the star.

  1. $\displaystyle 30{ kms }^{ -1 }$
  2. $\displaystyle 60{ kms }^{ -1 }$
  3. $\displaystyle 90{ kms }^{ -1 }$

  4. None of these
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Answer is B.

The apparent change in the wavelength is given as follows.
$\displaystyle \frac { \Delta \lambda  }{ \lambda  } =\frac { v }{ C } ,Hence\quad V=\frac { \Delta \lambda  }{ \lambda  } C$
$\displaystyle =\frac { 0.02 }{ 100 } \times 3\times { 10 }^{ 8 }{ ms }^{ -1 }=60{ kms }^{ -1 }$.
Hence, the velocity of the star is 60 km/s.

Multiple choice physics option a: relativity the nature of light speed of light and optical density introduction to light

An electromagnetic wave of frequency $\mathrm{v}=3.0$ MHz passes from vacuum into a dielectric medium with permittivity $\epsilon =4.0$. Then : 

  1. wavelength is doubled and frequency remains unchanged

  2. wavelength is doubled and frequency becomes half

  3. wavelength is halved and frequency remains unchanged

  4. wavelength and frequency both remain unchanged
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

$f=3.0mHz$

$E=4.0$

$C=fZ$

C is the speed of light into the medium

${ C } _{ 0 }=f _{ i }{ Z } _{ i }  C=\dfrac { { C } _{ 0 } }{ n } \\ C={ f } _{ f }{ Z } _{ f }  n=\sqrt { \dfrac { \varepsilon }{ { \varepsilon } _{ 0 } } } \\ C=\dfrac { { C } _{ 0 } }{ 2 } $

Now since frequency depends on source

Hence ${ f } _{ f }={ f } _{ i }\\ { Z } _{ f }=\dfrac { { Z } _{ i } }{ 2 } $

Speed and wavelength will be halved

Multiple choice physics option a: relativity the nature of light speed of light and optical density introduction to light

A certain color of light towards the purple end of the visible spectrum has a wavelength of $420\ nm$ in a vacuum.
What is the frequency of this light?
The speed of light in a vacuum is $3.00\times 10^8 m/s$

  1. $126 Hz$
  2. $1.26\times 10^{11}Hz$
  3. $7.1\times 10^{14}Hz$
  4. $1.4\times 10^{-15}Hz$
  5. $1.4\times 10^{-6}Hz$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Given :   $v = 3.00 \times 10^8$  m/s                 $\lambda = 420$ nm $ = 420 \times 10^{-9}$  m

$\therefore$ Frequency of the light       $\nu = \dfrac{v}{\lambda} = \dfrac{3.00 \times 10^8}{420 \times 10^{-9}} = 7.1 \times 10^{14}$  $Hz$

Multiple choice the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

Which of the following is not true for electromagnetic waves?

  1. They transport energy.

  2. They have momentum.

  3. They travel at different speeds in air depending on their frequency.

  4. They travel at different speeds in medium depending on their frequency.
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Electromagnetic wave have constant velocity in a particular medium and it is independant of frequency and it is equal to $V=\sqrt{\dfrac{1}{\mu _0 \epsilon _0}}=3 \times 10^8 \ m/s$ 

Option C

Multiple choice the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

An electromagnetic wave of frequency $\upsilon = 3$ MHz passes from a vacuum into a dielectric medium with permittivity $ = 4$, Then

  1. Wavelength and frequency both become half.

  2. Wavelength is doubled and frequency remains unchanged.

  3. Wavelength and frequency both remains unchanged.

  4. Wavelength is halfved and frequency remains unchanged.
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Frequency remains constant during refraction.

$v _{med}=\dfrac{1}{\sqrt{\mu _0 \epsilon _0 *4}}$=$\dfrac{c}{2}$

$\dfrac {\lambda _{med}}{\lambda _{air}} = \dfrac{v _{med}}{v _{air}}$

= $\dfrac{\dfrac{c}{2}}{c}$ =$\dfrac{1}{2}$
​$\therefore$ wavelength is halved and frequency remains unchanged.

Multiple choice the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

Select wrong statement from the following:
Electromagnetic waves

  1. are transverse

  2. travel with same speed in all media

  3. travel with the speed of light

  4. are produced by accelerating charge
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Electromagnetic waves are transverse and do not need a material medium to travel. 
The speed of all electromagnetic waves is equal to the speed of light. In fact this let to the conclusion that light is an electromagnetic wave.

Accelerating charge particles produce electromagnetic waves.
However the speed of electromagnetic waves changes with media as 
$v=\dfrac{c}{n}$
where n is the refractive index of the given medium.

Multiple choice the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

The wavelengths for the light of red and blue colours are roughly $7.8\times 10^{-7}m$ and $4.8\times 10^{-7}m$ respectively. Which colour has the greater speed in glass?

  1. Red light

  2. Violet light

  3. Yellow light

  4. All colors have same speed
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

When travelling through a medium, the frequency of light doesn't change. Hence, red light having the maximum wavelength will travel fastest through glass as speed of light is given by $v=f\lambda$ where; $f$ is the frequency and $\lambda$ is the wavelength of light travelling at speed $v$.

Multiple choice the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

What is the speed of Light $c$?

  1. $c=3\times 10^{-8} m s^{-1}$
  2. $c=3\times 10^8 km s^{-1}$
  3. $c=3\times 10^8 mm s^{-1}$
  4. $c=3\times 10^8 m s^{-1}$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its value is exactly $299792458$ meters per second.
That is, $c=3\times { 10 }^{ 8 }m/s$.