Answer:
We see the rainbow because of geometry of rain drops. Also rainbow is caused by reflection, refraction and dispersion of light in water droplets.
Rainbow is a semi-circular band of 7 colors that we can see in the sky. The spectrum is formed due to breakdown of white light in to its basic colors.
The light is the Sun's rays, they reflects from surface of water droplets. Whenever the light falls from sun is in the pure form of white light but when it reaches the surface of droplet at particular angle it breakdown in to spectrum of colors. This spectrum reaches the inner most point of the top of water droplet and gets reflected in to lower most part of droplet and thereby a spectrum refracted and gets dispersed, escapes from water droplet. This dispersed spectrum colors, we see as a rainbow.