Physics · Science General

Optics and Light Properties

2,019 Questions

Optics and light properties focus on the behavior of light, including reflection, refraction, dispersion, and polarization. This topic also covers the wave nature of light, illumination, and the functioning of optical instruments like microscopes. Questions on these concepts are common in general science sections of SSC, Railways, and State exams.

Reflection and mirrorsRefraction and mediumsLight wave theoriesPolarization and intensity

Optics and Light Properties Questions

Multiple choice physics refraction of light optical fibre the critical angle, total internal reflection and optical fibre total internal reflection

Critical angle of glass is $\theta _1$ and that of water is $\theta _2$. The critical angle for water and glass surface would be $(\mu _g=3/2, \mu _w=4/3)$

  1. less than $\theta _2$
  2. between $\theta _1$ and $\theta _2$
  3. greater than $\theta _2$
  4. less than $\theta _1$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

At critical angle. $\mu _{dense} \times sin \theta _ {dense} = \mu _{rare} \times sin 90°$


For glass - air: $(3/2) \times Sin \theta _ {1} = 1 => \theta _{1} = 41.81 ^\circ$
For water - air: $ (4/3) \times  Sin \theta _ {2} = 1 => \theta _{2} = 48.59^\circ$

For water- glass, glass is denser: $ (3/2) \times  Sin \alpha = (4/3)$ => $ \alpha = 62.72^\circ$

Multiple choice physics refraction of light optical fibre the critical angle, total internal reflection and optical fibre total internal reflection

A ray of light travelling in a transparent medium of refractive index $\mu$, falls on a surface separating the medium from air at an angle of incidence of $45^o$ . For which of the following value of $\mu$ the ray can undergo total internal reflection ?

  1. $\mu = 1.33$
  2. $\mu = 1.40$
  3. $\mu = 1.50$
  4. $\mu = 1.25$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

For total internal reflection, 
sin i > sin c 
where, i = angle of incidence, C = critical angle 


But, $sin \, C \, = \, \dfrac{1}{\mu} \,\, \therefore \, sin \, i \, = \, \dfrac{1}{\mu} \,\, or \,\, \mu \, = \, \dfrac{1}{sin \, i}$

$\therefore \, \mu \, > \, \dfrac{1}{sin \, 45} \,\, or \,\, \mu \, > \, \sqrt 2 \,\, (i \, = \, 45 \, Given)$

Hence, option (c) is correct.

Multiple choice physics refraction of light optical fibre the critical angle, total internal reflection and optical fibre total internal reflection

A ray of light travelling in a transparent medium of refractive index $\mu $, falls on a surface separating the medium from air at an angle of incidence of $45^o$. For which of the following value of $\mu $ the ray can undergo total internal reflection?

  1. $\mu =1.33$
  2. $\mu =1.40$
  3. $\mu =1.50$
  4. $\mu =1.25$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Given: The angle of incidence of the ray is $45^\circ$.


To find: The refractive index of the medium for Total Internal Reflection to occur.

For total internal reflection, the angle of incidence should be greater than the critical angle.
The critical angle for the ray can be given by:
$sin\ C=\dfrac{1}{\mu}$

$C=sin^{-1}\dfrac{1}{\mu}$

and $sin\ C>sin\ 45^\circ$

$\therefore sin 45>\dfrac{1}{\mu}$
$\mu>\dfrac{1}{sin\ 45}$

$\mu>1.41$

Option $(C)$ is correct.

Multiple choice physics refraction of light optical fibre the critical angle, total internal reflection and optical fibre total internal reflection

Critical angle for light going from medium (i) to (ii) is $\theta $. The speed of light in medium (i) is v, then the speed of light in medium (ii) is

  1. $v\left( 1-\cos { \theta } \right) $
  2. $\dfrac { v }{ \sin { \theta } } $
  3. $\dfrac { v }{ \cos { \theta } } $
  4. $\dfrac { v }{ \left( 1-\sin { \theta } \right) } $
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
Critical angle$=\theta$
Speed of light in medium(i)$=$v
$\sin\theta=\cfrac{1}{\mu}$
$\mu=\cfrac{1}{\sin\theta}$
$\cfrac{\mu _{2}}{\mu _{1}}=\cfrac{v _{2}}{v _{1}}$
where, $\mu _{2}=\mu$, $\mu _{1}=1$, $v _{1}=v$
$\Rightarrow \cfrac{\mu}{1}=\cfrac{v _{2}}{v}$
$\Rightarrow \cfrac{v _{2}}{v _{1}}=\cfrac{1}{\sin\theta}$
$\Rightarrow v _{2}=\cfrac{v}{\sin\theta}$

Multiple choice physics refraction of light optical fibre the critical angle, total internal reflection and optical fibre total internal reflection

If a solid transparent object has an refractive index of $2.90$ and a clear liquid has a refractive index of $1.45$ then, which of the following must be true for total internal reflection to occur at the interface between these two media?

  1. Incident beam originates in the solid at an angle of incidence greater than $30^o$
  2. Incident beam originates in the liquid at an angle of incidence greater than $30^o$
  3. Incident beam originates in the solid at an angle of incidence greater than $60^o$
  4. Incident beam originates in the liquid at an angle of incidence greater than $60^o$

  5. Total internal reflection cannot occur
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

There are two necessary conditions for total internal reflection

 (i) The light beam must go from denser to rarer medium.
(ii) Angle of incidence must be greater than critical angle.
here solid medium has greater refractive index therefore it is denser medium and beam must go from solid medium to liquid medium i.e. it must originates in solid medium.
   now angle of incidence is given by  
         $\sin C= \frac{{\mu} _{rarer}}{{\mu} _{denser}}=\frac{1.45}{2.90}=0.5000$
    or   $\sin C=\sin30$
    or  $C=30$
therefore angle of incidence must be greater than $30$  degree.

Multiple choice physics refraction of light optical fibre the critical angle, total internal reflection and optical fibre total internal reflection

The index of refraction for diamond is $2.42$. For a diamond in the air (index of refraction = $1.00$), what is the smallest angle that a light ray inside the diamond can make with a normal and completely reflect back inside the diamond (the critical angle)?

  1. $90^o$
  2. $45^o$
  3. $68^o$
  4. $66^o$
  5. $24^o$
Reveal answer Fill a bubble to check yourself
E Correct answer
Explanation
Given :   $n _{a} =1.00$                $n _d = 2.42$
For total internal reflection to occur, the incidence angle must be greater than the critical angle.
Critical angle  of diamond     $\theta = sin^{-1} \bigg( \dfrac{n _a}{n _d} \bigg)$ $= sin^{-1} \bigg( \dfrac{1.00}{2.42} \bigg)  = 24^o$
Thus the smallest angle for total internal reflection to occur in diamond is  $24^o$.
Multiple choice physics refraction of light optical fibre the critical angle, total internal reflection and optical fibre total internal reflection

The critical angle for a medium with respect to air is $45^o$. The refractive index of that medium with respect to air is

  1. $\dfrac{\sqrt 3}{2}$
  2. $\dfrac{2}{\sqrt 3}$
  3. $\sqrt 2$
  4. $\dfrac{1}{\sqrt 2}$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Critical angle $\theta _c=45^o$

For total internal reflection 
$n _{medium}sin\theta _c=n _{air}$ , the refractive index of air is 1
$n _{medium}sin\theta _c=1$
$n _{medium}=\dfrac{1}{sin45}=\sqrt{2}$

Multiple choice physics refraction of light optical fibre the critical angle, total internal reflection and optical fibre total internal reflection

A green light is incident from the water to the air - water interface at the critical angle ($\theta$). Select the correct statement.

  1. The spectrum of visible light whose frequency is more than that of green light will come out to the air medium.

  2. The entire spectrum of visible light will come out of the water at various angles to the normal.
  3. The entire spectrum of visible light will come out of the water at an angle of $90^o$ to the normal.

  4. The spectrum of visible light whose frequency is less than that of green light will come out to the air medium.
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

As frequency of visible light increases refractive index increases. With the increase of refractive index critical angle decreases. So that light having frequency greater than green will get total internal reflection and the light having frequency less than green will pass to air.

Multiple choice physics refraction of light optical fibre the critical angle, total internal reflection and optical fibre total internal reflection

A glass cube of refractive index $1.5$ and edge $1cm$ has tiny black spot at its center. A circular dark sheet is to be kept symmetrically on the top surface so that the central spot is not visible from the top.Minimum radius of the circular sheet should be (Given $\dfrac{1}{\sqrt{2}}=0.707,\dfrac{1}{\sqrt{3}}=0.577,\dfrac{1}{\sqrt{5}}=0.447)$

  1. $0.994cm$
  2. $0.447cm$
  3. $0.553cm$
  4. $0.577cm$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
$r\sin i=1.\sin 90^o$

$\sin i=\dfrac{1}{r}=\dfrac{2}{3}$

$\tan (90-i)=\dfrac{1}{2x}$

$\cot i =\dfrac{1}{2x}$

$\dfrac{\sqrt{3^2-2^2}}{x}=\dfrac{1}{2x}=D$

$x=\dfrac{1}{\sqrt{5}}=0.447 cm$
Multiple choice physics refraction of light optical fibre the critical angle, total internal reflection and optical fibre total internal reflection

A vertical pencil of rays comes from bottom of a tank filled with a liquid. When it is accelerated with an acceleration of 7.5 m/s$^2$, the ray is seen to be totally reflected by liquid surface. What is minimum possible refractive index of liquid?

  1. slightly greater than 4/3

  2. slightly greater than 5/3

  3. slightly greater than 1.5

  4. slightly greater than 1.75
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$tan\theta=a/g=37^o\Rightarrow \theta _c<37^o $


$sin\theta _c=sin 37\Rightarrow \mu>5/3$

Multiple choice physics refraction of light optical fibre the critical angle, total internal reflection and optical fibre total internal reflection

If a ray of light passes from a denser medium to a rarer medium in a straight line, the angle of incidence must be ______

  1. $0^o$
  2. $30^o$
  3. $60^o$
  4. $90^o$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

When light travels from one medium to another, it only continues in a straight line without refraction if the angle of incidence is 0 degrees (normal incidence).

Multiple choice physics refraction of light optical fibre the critical angle, total internal reflection and optical fibre total internal reflection

If the critical angle for the medium of prism is $C$ and the angle of prism is $A$, then there will be no emergent ray when-

  1. $A< 2C$
  2. $A=2C$
  3. $A> 2C$
  4. $A\ge 2C$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
Let $r _1$ be the angle of refraction at first face
and $r _2$ be the angle of refraction at second face.
as $r _1+r _2=A$
 thus, $r _1+r _2>2C.$
 This is possible if $r _2>C$
 ( as if $r _1>C$ incident ray would not enter the prism). 
$r _2>C$ implies no emergent ray.
Hence, Option $C$ is correct.
Multiple choice physics refraction of light optical fibre the critical angle, total internal reflection and optical fibre total internal reflection

A ray of light traveling in a transparent medium falls on a surface separating the medium from air, at an angle of $ { 45 }^{ \circ  }$. The ray undergoes total internal reflection. if 'n' is the refractive index of the medium with respect to air, select the possible values of 'n' from the following.

  1. 1.3

  2. 1.4

  3. 1.5

  4. 1.6
Reveal answer Fill a bubble to check yourself
C,D Correct answer
Explanation

The angle of incidence of all the rays is $ { 45 }^{ \circ  }$ at the hypotenuse. For a critical angle of $ { 45 }^{ \circ  }$, the refractive index must be 
$ { \left( \sin { { 45 }^{ \circ  } }  \right)  }^{ -1 }=\sqrt { 2 } =1.414$

Multiple choice physics refraction of light optical fibre the critical angle, total internal reflection and optical fibre total internal reflection

When ray of light enters from one medium to another its velocity in second medium becomes double. the maximum value of angle of incidence so that total internal reflection may not take place will be

  1. $ { 60 }^{ \circ }$
  2. $ { 180 }^{ \circ }$
  3. $ { 90 }^{ \circ }$
  4. $ { 30 }^{ \circ }$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Velocity becomes double so $\mu _{r}=2$


critical angle is $sin^{-1}\dfrac{1}{\mu _{r}}=sin^{-1}\dfrac{1}{2}=30^{\circ}$.