Tag: physics

Questions Related to physics

The difference between angle of minimum deviation for violet and red rays in the spectrum of white light from a prism is $2^0$. If the angle of minimum deviation of the mean ray is $48^0$, the dispersive power of the material of the prism is

perceive colours resolution of optical instruments lenses option c: imaging physics

  1. $24^0$

  2. $48^0$

  3. 0.0416

  4. 0.0832

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Correct Option: D

The focal length of a thin convex lens for red and blue rays are $100\ cm$ and $96.8\ cm$ respectively. The dispersive power of the material of the lens is

perceive colours resolution of optical instruments lenses option c: imaging physics

  1. 0.0325

  2. 0.825

  3. 0.968

  4. 0.98

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Correct Option: A

If a glass prism is dipped in water, its dispersive power

perceive colours resolution of optical instruments lenses option c: imaging physics

  1. increases

  2. decreases

  3. does not change

  4. may increase or decrease depending on whether the angle of the prism is less than or greater than $60^o$

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Correct Option: A
Explanation:

Dispersive power of a prism $=\dfrac{\mu _{v}-\mu _{r}}{\mu-1}$


Here the refractive indices are with respect to the medium in which prism is kept.

Hence when in water, dispersive power $=\dfrac{\dfrac{\mu _{v}}{\mu _{water}}-\dfrac{\mu _{r}}{\mu _{water}}}{\dfrac{\mu}{\mu _{water}}-1}$
$=\dfrac{\mu _{v}-\mu _{r}}{\mu-\mu _{water}}$

Hence dispersive power increases when prism is dipped in water.

Two thin prisms of flint glass, with refracting angles of $6^o$ and $8^o$ respectively, possess disperse powers in the ratio 

perceive colours resolution of optical instruments lenses option c: imaging physics

  1. 4 : 3

  2. 3 : 4

  3. 1 : 1

  4. 9 : 16

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Correct Option: C
Explanation:

Dispersive power = $\dfrac{\mu _v-\mu _r}{\mu-1}$ 

Since, it does not depend on angle of prism, dispersive power f both prism will be same.
Therefore, C is correct option.

Refractive index of glass for red and violet colours are $1.64$ and $1.66$ respectively. Dispersive power of the prism is :

perceive colours resolution of optical instruments lenses option c: imaging physics

  1. $0.3$

  2. $3.3$

  3. $1.33$

  4. $.03$

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Correct Option: D

The refractive index of flint glass for blue line is 1.6333 and red line is 1.6161, then dispersive power of the glass is :

perceive colours resolution of optical instruments lenses option c: imaging physics

  1. 0.0276

  2. 0.276

  3. 2.76

  4. 0.106

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Correct Option: A
Explanation:

Given :     $n _b = 1.6333$             $n _r = 1.6161$

Refractive index for yellow light    $n _y = \dfrac{n _b+n _y}{2}$
$\therefore \ n _y = \dfrac{1.6333+1.6161}{2} = 1.6247$
Dispersive power     $w = \dfrac{n _b-n _r}{n _y - 1} = \dfrac{1.6333-1.6161}{1.6247-1} = 0.0276$

Using the following data,choose the correct option:
                     C        D        F      
 Crown   1.5145   1.5170  1.5230   
FLINT    1.6444    1.6520  1.6637                  

perceive colours resolution of optical instruments lenses option c: imaging physics

  1. The dispersive power for crown glass is 0.1644

  2. The dispersive power for flint glass is 0.029601

  3. The dispersive power for crown is 0.01644

  4. The dispersive power for flint glass is 1.29601

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Correct Option: B,C

If D is the deviation of a normally falling light beam on a thin prism of angle a and $\delta$ is the dispersive power of the same prism then

perceive colours resolution of optical instruments lenses option c: imaging physics

  1. D is independent of A

  2. D is independent of refractive Index

  3. $\delta$ is independent of refractive index

  4. $\delta$ is independent of A

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Correct Option: D
Explanation:

Using relation $ D=\left( { \mu  } _{ v }-{ \mu  } _{ r } \right) A$


and $ \delta =\dfrac { { \mu  } _{ v }-{ \mu  } _{ r } }{ { \mu  } _{ y }-1 } $ we get,

$ \delta$ is independent of $A$

If a glass prism is dipped in water, its dispersive power

perceive colours resolution of optical instruments lenses option c: imaging physics

  1. increases

  2. decreases

  3. does not change

  4. may increase or decreases depending on whether the angle of the prism is less than or greater than $ { 60 }^{ \circ }$

Show answer
Correct Option: B
Explanation:

Glass prism works on the principle that lights with different colours travels with different speed in solid and liquid medium. So when a glass prism is dipped in water the light that reaches it is already some what dispersed by water, and the prism does still more. Hence its dispersive power decreases.

A thin prism deviates blue and red rays through 10$^{\circ}$ and 6$^{\circ}$respectively. Another prism deviates same colours through 8$^{\circ}$ and 4.5$^{\circ}$ respectively.The ratio of dispersive powers of the prisms is :

perceive colours resolution of optical instruments lenses option c: imaging physics

  1. 5:4

  2. 4:5

  3. 25:28

  4. 5:28

Show answer
Correct Option: C
Explanation:

For a thin prism,

Dispersive power, $w = (\delta _{v}-\delta _{r}) / \delta _{y}$

Also, $\delta _{y} = (\delta _{v}+\delta _{r})/2$

Ratio of dispersive power,
$w _{1} : w _{2} = [(10-6) * 2 / (10+6)] : [(8-4.5)*2)/ (8+4.5)]$

=> $w _{1} : w _{2} = 25:28$