Write a note on rainbow.

(N/A) rainbow is a natural phenomenon caused by the dispersion,refraction,and reflection of sunlight by spherical water droplets in the atmosphere.
Conditions for observing a rainbow: The sun must be shining in one part of the sky (e.g.,near the western horizon) while it is raining in the opposite part (e.g.,eastern horizon). An observer can see a rainbow only when their 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 wavelengths of light (red) are bent the least,while shorter wavelengths (violet) are bent the most.
These component rays strike the inner surface of the water drop and undergo internal reflection if the angle of incidence is appropriate. The reflected light is refracted again as it emerges from the drop. It is observed that violet light emerges at an angle of $40^{\circ}$ relative to the incoming sunlight,and red light emerges at an angle of $42^{\circ}$. For other colours,the angles lie between these two values.