Write a note on rainbow.

Explanation

The rainbow is an example of the dispersion of sunlight by the water drops in the atmosphere.
This is a phenomenon due to combined effect of dispersion, refraction and reflection of sunlight by spherical water droplets of rain.
The conditions for observing a rainbow are that the sun should be shining in one part of the sky (say near western horizon) while it is raining in the opposite part of the sky (say eastern horizon). An observer can therefore see a rainbow only when his back is towards the sun.
Formation of rainbows: Sunlight is first refracted as it enters a raindrop which causes the different wavelengths (colours) of white light to separate.
Longer wavelength of light (red) are bent the least while the shorter wavelength (violet) are bent the most.
These component rays strike the inner surface of the water drop and get internally reflected if the angle between the refracted ray and normal to the drop surface is greater then the critical angle `48^@`.
The reflected light is refracted again as it comes out of the drop as shown in the figure.

It is found that the violet light emerges at an angle of `40^@` related to the incoming sunlight and red light emerges at an angle of `42^@`. For other colours, angles lie in between these two values.
Red light from drop 1 and violet light from dro 2 reach the observer.s eye. The violet from dro 1 and red light from drop 2 are directed at lev above or below the observer. Thus the observ sees a rainbow with red colour on the top ar violet on the bottom,
Thus, the primary rainbow is a result of thre step process, that is refraction, reflection an refraction.
When light rays undergoes two interni reflections inside a raindrop, instead of one as i the primary rainbow a secondary rainbow formed as shown in figure (c).

The intensity of light is reduced at the second reflection and hence the secondary rainbow in fainter than the primary rainbow.
Further, the order of the colours is reversed in it.