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21. Why do stars twinkle but not the planets?

The density of the atmosphere is not uniform as it goes on decreasing with the distance from the earth. The rays coming from the distant stars have to pass through the various mediums in which the light gets refracted due to the variation of densities. Also the atmosphere in the earth is not stationary due to which the position of the star changes much faster from the earth. Hence, the change in the position of the star creates the twinkling of the star.

But the distance betweens the planet and the earth is much more less than the distance between the earth and the star. So, there is negligible change in the position of the planet from the earth due to which it does not appear to be twinkling at the sky.

 
22. Why does a crack in glass plane appear shining when viewed from a suitable direction?

In a crack glass, the air replaces the cracked volume. When it is observed from the suitable direction, the light of it has to pass from two medium from the glass to the air i.e. from the denser to the rarer medium. during their journey they make incidence angle greater than the critical angle due to which the total internal reflection process occurs. Hence these rays appeared to be shining when viewed from a suitable directions. So, crack appears to shine.
 
 
23. If you see a fish in clear water, how should you aim to shoot it? Explain.

As the light rays coming from a fish  passes through two medium i.e. from denser to rarer medium. As the light passes from denser to rarer medium, according to the law of refraction, it goes away from the normal. Due to this the fish appear to be nearer to the surface i.e. it appears in its apparent position. In order to shoot it we should aim at a position below than the apparent position which is the actual position of the fish.
 
24. Why does powdered glass appear white and opaque but becomes transparent when water is poured over it? Explain.

As the surface of a powdered glass is not regular but is rough and irregular so, irregular or diffused reflection occurs due to which the  powdered glass appears white and opaque.

But when the water is poured over it, almost all rays are refracted instead of reflection due to the refractive index of water ( = 1.33) and glass ( = 1.5) being nearly equal. So, due to the formation of the media the light undergoes the refraction phenomena instead of the reflection due to which the powdered glass appears transparent when water is poured in it.

 
25. What does a fish see when it looks towards the sky?
 
When the fish looks towards the sky it observes the sky portion in circular hole where the range of the angle is critical angle and beyond that circle it observes the objects of water due to the total internal reflection. If the critical angle is c and the fish is at height h from the water surface then

          

Now,           

        

where, r is the radius of the circular hole observed by the fish

            is the refractive index of water.

              Or, r = h(tanc)

                   

26. A fish under water is viewing a tree obliquely on the bank of river. Does the tree look shorter or taller than its actual height?

When a tree in the air medium i.e. rarer medium is viewed by a fish under water i.e. from the denser medium, the apparent height of the tree is observed by a fish which is greater than the real height of the tree. It is due to the phenomena of refraction when the light traveled from the denser to rarer medium, it goes away from the normal. So, the tree looks taller than its actual height.
 

27. Why does a diamond sparkle with a great brilliancy?

The refractive index of the diamond is very high and its critical angle is about . The faces of the diamond are cut in such a way that when a light enters into it, the angle of incident is always greater than the critical angle, which results in the multiple total internal reflection. So, due to this the diamond sparkle with great brilliancy.
 
 
28. Why does a clear pool of water appear to be shallower than it actually is?

Due to the refraction of light, the light rays coming from the bottom of the pool bends away from the normal. Hence the light rays actually coming from the depth of the pool appears as coming from the lower depth than actual depth. So, the depth of the pond appears slightly raised due to which a clear pool of water appear to be shallower than it actually is.
 
 
29. If a plane glass slab is placed on letter of different colors, then violet colored letters appear more and more raised up. Why?
The vertical distance by which a body or anything rises as the introduction of a refracting medium is given by
Relation for height raised during real and apparent depth
The refractive index in turn is given by

 
30. Why do the trees and houses on the other side of an open ground appear to be shaking during the summer noon?

During the summer noon, the air in contact with ground gets heated. When temperature rise, the density of air is decrease as the density varies inversely with the temperature. as a result of which there is a movement of hot air towards upward and the cold air downward creating the difference in the densities. So, when light rays pass via it, the path of light rays are changed due to refraction. Due to the fluctuations of air above the ground produce shaking appearance of the tree and houses on the side of an open ground.
 
 
31. How does the refraction affect the length of the day?

If there is no atmosphere on the earth surface, the length of the day decreases by 4 minutes i.e. 2 minutes in morning & 2 minutes in evening. Hence, Due to refraction of light through the atmosphere, the length of the day increases by 4 minutes.
 
 
32. A coin placed at the bottom of a beaker containing water seems to be raised. Explain why?

When light rays of a coin from the bottom of water is observed by the observer in the air then the presence of the two media results in the refraction of the light. When light rays travel from the water (denser medium ) to air (rarer medium), bends away from the normal. So, the refracted rays coming from a coin is observed at its apparent position rather than its real position. Hence, a coin placed at the bottom of a beaker containing water seems to be raised.
 
 
33. Why does the sun look a little oval when it is at the horizon?

The refraction of the light due to the variation in the density of the air in the atmosphere is the main cause for the oval shape of the sun when it is at the horizon. The magnitude of refraction increases with decreasing the altitude. As a result of which the lower portion of the sun are raised much than the upper position. So, the vertical diameter is shortened more than its horizontal diameter. Hence the sun acquires the oval shape at the horizon.
 
 
34. Swimming pools are always deeper than they look. Why?

When the swimming pool is observed, the light rays of the foot of the pool have to pass through the two media from water to air i.e. from denser to rarer medium. As the passes from denser to rarer medium, it goes away from the normal. Hence the original or real depth of the pool is observed at its apparent position where it is raised. So, swimming pools are always deeper than they look.
 
 
35. Why does an air bubble in a jar of water shine brightly?

Light rays hitting the edges of the air bubble make the incident angle always greater than the critical angle which results in the total internal reflection. Due to it, an air bubble in a jar shine brightly.

 

 

36. The sun is visible a even before the actual sunrise and after actual sunset. Explain.

The density of air decreases as the distant is increased from the earth in the atmosphere. The magnitude of refraction increases with decrease in the height. Due to this, the lower portion of the sun at the time of sun set and sunrise are raised more than the upper portion. So, the vertical diameter of the sun is shortened more than the horizontal diameter. As a result, the sun is visible a even before the actual sunrise and after actual sunset.

 
37. Why one prefers a black umbrella even in summer?

A black umbrella absorbs all the radiations of the sunlight  and it provide better shade,  while a white umbrella is not preferred because a part of the sunlight may penetrate through it which may be harmful, So, one prefers a black umbrella even in summer.
 
 
38. A stick partially dipped in water seems to be bent. Why?

When light rays passes from denser medium to rarer medium, then rays bent away from the normal. Due to the refraction of the light, the rays coming from the bottom of the stick dipped goes away from the normal which results in the bending of a stick when dipped in water.
 
 
39. Why do in summer roads often appear to be covered with water when seen from a distance? Explain.
 
In the pitch road due to heat of the sun, the layer near the road becomes hot and it behaves as a rarer medium. And due to the total internal reflection, a mirage ( which is an optical illusion ) is seen in the pitch road during the summer. A ray of light coming from the road passes successively from denser to rarer layers and bends away from the normal. When the angle of incident is greater than the critical angle, the ray gets totally internally reflected and reaches the eyes of the observer who sees the roads covered with water.  Hence, in summer roads are often appears to be covered with water when seen from a distance.

 

40. Which one of blue and red lights will be deviated more by a prism and why?
 
The deviation produced by a small angled prism is given by the relation:

 

where, A is the angle of the prism and 

is the refractive index of the material of the prism. As the refractive index increases with decrease in the wavelength, the blue light has more refractive index than that of red light. The larger the refractive index, the greater is the deviation. Hence, blue light will be deviated more than red light by a prism.

 

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