The spectral colors of the rainbow are because of dispersive refraction. The refraction of a medium tells us how fast light travels in that medium.
It is the ratio of the speed of light to the speed of light in this medium. The bending of a light as it crosses into a medium is determined by its refractive index, or how much the light is slowed down in the new medium.
When the sun shines from behind us into the rain, incident rays of light enter the drop and are refracted inwards. Colors that make up white light are separated according to frequency when they pass through glass. If the glass bends the light twice you can see the separated colors more easily. That’s called
dispersion.
