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

The speed of light in two media  I and II are $2.2\times 10^8 m/s$ and $ 2.4\times { 10 }^{ 8 }{ m }/{ s }$ respectively. The critical angle for light refracting from I to II medium will be

  1. $\sin^{-1}( { \frac { 12 }{ 11 } } )$
  2. $ \sin^{-1} ({ \frac { 11 }{ 12 } } )$
  3. $ \sin^{-1} ({ \frac { 12 }{ 24 } } )$
  4. $ \sin^{-1}( { \frac { 12 }{ 21 } }) $
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$\mu _{r}=\dfrac{2.4}{2.2}$


critical angle is $sin^{-1}\dfrac{1}{\mu _{r}}=sin^{-1}\dfrac{11}{12}$

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

An optical fiber ($\mu=1.72$) has a coating of glass ($\mu=1.5$). The critical angle for total internal reflection is

  1. $ { sin }^{ -1 }\left( \dfrac { 75 }{ 86 } \right) $
  2. $ { sin }^{ -1 }\left( \dfrac { 86 }{ 75 } \right) $
  3. $ { sin }^{ -1 }\left( 0.8 \right) $
  4. $ { sin }^{ -1 }\left( 0.82 \right) $
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The critical angle for total internal reflection is given as $sin\theta _c=\dfrac{\mu _2}{\mu _1}$, where $\mu _1$ is the refractive index of the denser medium.
Thus we get 


$sin\theta _c=\dfrac{1.5}{1.72}$

or  $\theta _c=sin^{-1}\dfrac{75}{86}$.

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

If the refractive index of water is 4/3 and that of glass is 5/3, then the critical angle of light entering from glass into water will be

  1. $ \sin^{-1} ({ \dfrac { 4 }{ 5 } } )$
  2. $ \sin^{-1} ({ \dfrac { 3 }{ 4 } }) $
  3. $ \sin^{-1} ({ \dfrac { 1 }{ 2 } } )$
  4. $ \sin^{-1} ({ \dfrac { 2 }{ 3 } }) $
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
When a light ray travels from denser to rarer medium, then the angle of incidence for which the refracted ray becomes parallel to the interface$( \angle r = 90^{\circ} )$, is known as critical angle for that pair of media. 
Here, the denser medium is Glass $ \mu _g = \dfrac{5}{3} $
&
the rarer medium is water $ \mu _w = \dfrac{4}{3} $

Applying Snell's law at the interface of the two media, we have
$ \mu _g \times Sin\ i _c = \mu _w \times Sin\ r $

From definition, we know, 
$ \angle r = 90^{\circ} $

$ \Rightarrow \mu _g \times Sin\ i _c = \mu _w \times Sin\ 90^{\circ} $

$ \Rightarrow \dfrac{5}{3} \times Sin\ i _c =  \dfrac{4}{3} $
$  \Rightarrow Sin\ i _c = \dfrac{\Big( \dfrac{4}{3} \Big) }{\Big( \dfrac{5}{3} \Big)} $

$  \Rightarrow Sin\ i _c = \dfrac{4}{5} $

$  \Rightarrow i _c = Sin^{-1} \Big( \dfrac{4}{5} \Big) $ 


Hence, the correct answer is OPTION A.

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

The velocity of light in a medium is half its velocity in air. If a ray of light emerges from such a medium into air, the angle of incidence, at which it will be totally internally reflected, is

  1. $15^o$
  2. $30^o$
  3. $45^o$
  4. $60^o$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

max angle is critical angle , velocity is halved hence $\mu _{r}=2$


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

option $B$ 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 glass $(\mu=3/2)$ is incident on a horizontal glass-air surface at the critical angle $\theta _C$. If a thin layer of water $(\mu =4/3)$ is now poured on the glass-air surface, the angle at which the ray emerges into air at the water-air surface is

  1. $60^o$
  2. $45^o$
  3. $90^o$
  4. $180^o$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

critical angle is $sin^{-1}\dfrac{1}{\mu _{r}}=sin^{-1}\dfrac{2}{3}$
apply Snell's law at all boundaries

$\mu _{1}sin\alpha _{1}=\mu _{2}sin\alpha _{2}=\mu _{3}sin\alpha _{3}$ 

$\dfrac{3}{2}sin \theta _{c}=1 \times sin\theta$

$\theta=90$, 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 from a denser medium strikes a rarer medium at an angle of incidence $i$. The reflected and refracted rays make an angle of $\pi/2$ with each other. If the angles of reflection and refraction are $r$ and $r'$, then the critical angle will be

  1. $\tan^{-1}(\sin i)$
  2. $\sin^{-1}(\sin r)$
  3. $\sin^{-1}(\tan i)$
  4. $\sin^{-1}(\tan r)$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
as the reflected and refracted rays are 90 then the angle of refraction is $90-i$

according to Snell's law
$sini\mu _{r}=sin(90-i)$

$\mu _{r}=coti$

critical angle is $sin^{-1}\dfrac{1}{\mu _{r}}=sin^{-1}\dfrac{1}{coti}=sin^{-1}tani$
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 fish looks upward at an unobstructed overcast sky. What total angle does the sky appear to subtend? (Take refractive index of water is $\sqrt 2)$

  1. $180^o$
  2. $90^o$
  3. $75^o$
  4. $60^o$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
as the refractive index is $\sqrt{2}$ the critical angle is $sin^{-1}\dfrac{1}{\mu _{c}}=sin^{-1}\dfrac{1}{\sqrt{2}}=45^{\circ}$

 total angle the sky appear to subtend is $2\times 45 =90^{\circ}$

option $B$ 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 incidence of $45^o$. The ray undergoes total internal reflection. If n is the refractive index of the medium with respect to air, select the possible value(s) 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

$\mu _{min}=\dfrac{1}{sin45}=\sqrt{2}=1.414$


possible values of $\mu$ are $1.5$ and $1.6$ 

option $C,D$ are correct 

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

The critical angle for glass-air is $45^o$ for the light of yellow colour. State whether it will be less than, equal to, or more than $45^o$ for  blue light?

  1. more than $45^o$
  2. less than $45^o$
  3. same as $45^o$

  4. cant say
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

According to Snell's Law: $\dfrac{sin i _c}{sin r}=\dfrac{n _2}{n _1}$ and for critical angle of incidence $r=90^o$ and $n _2=1$


Blue light has higher refractive index than yellow light hence it's critical angle will be lower than yellow light.

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

The critical angle for glass-air is $45^o$ for the light of yellow colour. State whether it will be less than , equal to, or more than $45^o$ for  red light?

  1. more than $45^o$
  2. less than $45^o$
  3. same as $45^o$

  4. can't say
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

According to Snell's Law: $\dfrac{sin i _c}{sin r}=\dfrac{n _2}{n _1}$ and for critical angle of incidence $r=90^o$ and $n _2=1$


Red light has lower refractive index than yellow light hence it's critical angle will be higher than yellow light.

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

State the approximate value of the critical angle for (a) glass-air surface, (b) water-air surface (Given $\mu _{glass} =1.5, \mu _{water}=1.33$)

  1. (a) $49^o$, (b)$49^o$
  2. (a) $42^o$, (b)$42^o$
  3. (a) $42^o$, (b)$49^o$
  4. (a) $49^o$, (b)$42^o$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Critical angle is the angle of incidence from denser to rarer medium for which the angle of refraction is $%%90^o$
These angles for glass-air interface is $42^o$ and water-air interface is $49^o$

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

The critical angle for light going from medium X into medium Y is $\theta$. The speed of light in medium X is v, then speed of light in medium Y is

  1. $v(1 - cos \theta)$
  2. $v/sin \theta$
  3. $v/ cos \theta$
  4. $v cos \theta$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
critical angle condition:

${ \mu  } _{ X }\sin { \theta  } ={ \mu  } _{ Y }$

Given, speed of light in medium X is v

To find: speed of light in medium Y

$\dfrac { { \mu  } _{ Y } }{ { \mu  } _{ X } } =\sin { \theta  }$

Also, speed of light in medium $Y$ is $=\dfrac{v}{\mu _{rel}}=v\dfrac { { \mu  } _{ X } }{ { \mu  } _{ Y } }$

$=\dfrac { v }{ \sin { \theta  }  }$
Multiple choice physics refraction of light optical fibre the critical angle, total internal reflection and optical fibre total internal reflection

Light takes $t _1$ sec to travel a distance 'x' in vacuum and the same light takes $'t _2'$ sec to travel 10 cm in a medium. Critical angle for corresponding medium will be

  1. $\displaystyle sin^{-1} \left ( \frac{10 t _2}{t _1 x} \right )$
  2. $\displaystyle sin^{-1} \left ( \frac{t _2 x}{10 t _1 } \right )$
  3. $\displaystyle sin^{-1} \left ( \frac{10 t _1}{t _2 x } \right )$
  4. $\displaystyle sin^{-1} \left ( \frac{t _1 x}{10 t _2 } \right )$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

$\displaystyle c = \frac{x}{t _1} , v = \frac{10}{t _2}$

$\displaystyle sin  i _c = \frac{1}{\mu} = \frac{v}{c} = \frac{10}{t _2} \times \frac{t _1}{x}$

$\Rightarrow     i _c = sin^{-1} \left ( \dfrac{10  t _1}{t _2 x} \right )$

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

A ray of light is travelling from glass to air. (Refractive index of glass $=1.5$). The angle of incidence is $50^o$.The deviation of the ray is

  1. $0^o$
  2. 80$^o$
  3. $50^o - sin^{-1} \displaystyle \left [ \frac{sin 50^o}{1.5} \right ]$
  4. $sin^{-1} \displaystyle \left [ \frac{sin 50^o}{1.5} \right ] - 50^o$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$^a\mu _g = 1.5$


$\therefore 1.5 = cosec  C$ 

Or, $C = 42^0$. 

Critical angle for glass $= 42^0$. Hence a ray of light incident at $50^0$ in glass medium undergoes total internal reflection. $\delta$ denotes the deviation of the ray.

$\delta = 180^o - (50^o + 50^o) $ or $\delta = 80^o$.

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

A light ray is incident at an angle ${30}^{o}$ on a transparent surface separating two media. If the angle of refraction is ${60}^{o}$ then critical angle is

  1. $\sin ^{ -1 }{ \left( \cfrac { 1 }{ \sqrt { 3 } } \right) } $
  2. $\sin ^{ -1 }{ \left( \sqrt { 3 } \right) } $
  3. $\sin ^{ -1 }{ \left( \cfrac { 2 }{ 3 } \right) } $
  4. ${45}^{o}$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Snell's law gives ${\mu} _{1} \sin {{30}^{o}}={\mu} _{2} \sin {{60}^{o}}$ but critical angle



${\theta} _{c}=\sin ^{ -1 }{ \left( \cfrac { { \mu  } _{ 2 } }{ { \mu  } _{ 1 } }  \right)  } =\sin ^{ -1 }{ \left( \cfrac { \sin { { 30 }^{ o } }  }{ \sin { { 60 }^{ o } }  }  \right)  } =\sin ^{ -1 }{ \left( \cfrac { 1 }{ \sqrt { 3 }  }  \right)  } $