​COLOURFUL WORLD 

Rainbow Formation:- 

It is a natural spectrum appearing in the sky after a rain shower. A rainbow is always formed in a direction opposite to that of the Sun.

Three phenomena which are involved in rainbow formation are:-

• refraction
• dispersion and
• total internal reflection

Atmospheric Refraction:- 

The refraction of sunlight by different layers of atmosphere is called atmospheric refraction.

Phenomena caused by atmospheric refraction:-

Apparent position of star:- 

The refractive index of earth’s atmosphere increases from top to bottom. So, the light coming from a star near the horizon has to travel from rarer to denser medium & it bends towards the normal. As a result the star appears higher than its actual position.

Twinkling of stars:- 

Since the physical conditions of the earth’s atmosphere are not constant, so the light from stars appears to come from different directions. This results in fluctuation of apparent position of star. Also the amount of light coming from stars vary due to changing refractive index of atmosphere. So the stars sometimes appear brighter and sometimes fainter. These two effects are responsible for twinkling of stars.

Advanced sunrise and delayed sunset:- 

When the sun is below horizon, the rays have to pass from rarer to denser medium. So rays bend towards normal. As a result, the sun appears higher than its actual position. So the sun rises two minutes earlier than actual sunrise and remains visible for about two minutes after the actual sunset.

Phenomena based on scattering of light:-

Tyndall effect:- The phenomenon of scattering of light by colloidal particles so that its path becomes visible is called tyndall effect.

Blue appearance of the sky:- 

The upper layer of atmosphere contains very fine particles. These particles are more effective in scattering light of shorter wavelength mainly upto blue end. These scattered light enter our eyes and the sky appears blue.

Reddish appearance of the sun at sunrise and sunset:- 

During sunset and sunrise, the sun is near horizon and the the sunlight has to travel larger distance in atmosphere. Due to this most of the light of shorter wavelengths are scattered away by the particles and light of longer wavelengths reaches our eyes. This gives reddish appearance of the sun at sunrise and sunset.