Human Eye and Colourful World

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Total Internal Reflection

When light enters obliquely from a denser medium to a rarer medium and the angle of incidence exceeds critical angle, the light reflects in the denser medium. This is called internal reflection.

Conditions necessary for Internal Reflection

1. Light should enter obliquely from a denser to a rarer medium.
2. The angle of incidence should exceed critical angle, the light reflects in the denser medium.
3. Critical angle : The angle of incidence for which the angle of refraction is 90º.

Rainbow

1. It is a natural spectrum appearing in the sky after rain showers.
2. Rainbow is observed in the direction opposite to the sun.
3. Three phenomena which are involved in rainbow formation are :
   
   1. Dispersion
   2. Refraction
   3. Internal reflection

Some water droplets remain suspended in air after the rain. These droplets behave as a glass prism. When light enters the raindrop, it first refracts and disperses. Then it reflects internally and again refracts as it comes out of the drop and the seven colours reach the eye of the observer in form of a rainbow.

Atmospheric Refraction: The refraction by different layers of the atmosphere is called atmospheric refraction.

1. The apparent flickering of objects placed behind a hot object or fire.
2. Stars near the horizon appear slightly higher than their actual position.
3. Advanced sunrise and delayed sunset.
4. Apparent flattering of sun’s disc.
5. Twinkling of stars.

The sun appears about two minutes earlier than actual sunrise and the sun remains visible for about two minutes after actual sunset.

When the sun is below the horizon, the rays have to pass from rarer to denser medium. So rays bend towards the normal. As a result, the sun appears higher than its actual position.

1. Stars emit their own light and they twinkle due to the atmospheric refraction of light.
2. Stars are very far away from the earth. Hence, they are considered as point sources of light.
3. When the light coming from stars enters the earth’s atmosphere, it gets refracted at different levels because of the variation in the air density at different levels of the atmosphere.
4. When the star light refracted by the atmosphere comes more towards us, it appears brighter than when it comes less towards us.
5. Therefore, it appears as if the stars are twinkling at night.