Explain the formation of rainbow.

i. Sunlight is comprised of seven colours namely violet, indigo, blue, green, yellow, orange & red (VIBGYOR). Every colour has different wavelength such as red has the maximum wavelength & violet has the least wavelength.

ii. As the sunlight travels in air (refractive index:1), all the colours travel with the same speed. But as soon as they travel in some medium, they get dispersed into seven colours as in this medium, the velocity of each colour changes. Rainbow is formed due to two phenomenons: reflection, dispersion & refraction.

iii. Rainbow is seen mainly in rainy season as the spherical droplets of water that are suspended in air cause the dispersion of sunlight.

iv. White light from the sun enters the top surface of the water droplet. Due to refraction, the sunlight disperses into seven colours & strikes the surface of droplet at point C.

v. At point C, the angle formed by incident ray is greater than the critical angle. Therefore, total internal reflection occurs at point C and the dispersed spectrum of lights are reflected at the bottom surface of the droplet.

vi. At point D, the spectrum again experiences refraction and as they are travelling from denser to rarer medium, the angle between red & violet increases as compared to the angle for the first refraction. There is one more condition, that is, bright rainbow is seen only when the angle between the incoming & the outgoing rays have the maximum angle of 42 degrees.