Raindrops act like tiny prisms — splitting white light into all the colours of the spectrum. As light rays pass through the raindrop, they are refracted, causing them to split into different colours. The rays bounce off the back of the raindrop and pass out, divided into all the colours seen in the rainbow.
As you stood looking at the rainbow, the sun was probably above and behind you. The rain was probably somewhere in front of you, if not landing on you at the time! Since the sun was behind and the rain in front of you, the sunlight must have been bouncing off the raindrops and reflecting into your eyes.
If the sunlight is primarily white light and is reflecting off the raindrops, why do we see colors in a rainbow? The colors are present because the sunlight is not only reflecting off the raindrops, it is also refracting and dispersing in the raindrops.
Notice in the drawing how the raindrop acts like a prism, splitting the white light from the sun (in the upper left) into its component colors. For simplification, I have only shown the red and blue beams.
But notice also how the raindrop acts like a mirror in that it reflects (some of) the refracted light back toward the sun. These refracted rays are the ones that you see as a rainbow.
When the white light from the sun hits the raindrop, the light is dispersed as it enters the raindrop, much like light is split as it passes through a prism. The separate colors are then reflected from the backside of the raindrop and exit, where they refract once again, due to the change of index of refraction between the water and the air.
The angles at which each color emerges is different (or else you wouldn’t see different colors!): red light emerges at 42° and blue light emerges at 40.6° relative to the incoming ray of sunlight.
Picture Credit : Google
