Dispersion and Rainbows
![[url=https://pixabay.com/en/rainbow-river-nature-landscape-2424647/]"Rainbow"[/url] by sharonjoy17 is in the [url=http://creativecommons.org/publicdomain/zero/1.0/]Public Domain, CC0[/url]
Rainbows occur due to dispersion, a topic that we will discuss in this section.](https://www.geogebra.org/resource/Wfhex4s8/HWRYNPuICe3CslNP/material-Wfhex4s8.png)
As mentioned in the section on refraction, the index of refraction for materials really depends on the wavelength of the light. As a general rule in nature, higher frequency light (the violet end of the spectrum) will always have a higher refractive index than light on the red end of the spectrum.
Because of the refractive index depending on wavelength - and therefore depending on color - we find that white light (which is comprised of all colors) will tend to split to some degree when refracting through glass or water or other materials. Such splitting is the basis of a rainbow or a glass prism, and is called dispersion.
Rainbows
Rainbows are a great example of several optical concepts all in one. A primary (not secondary) rainbow is formed when light refracts upon entry to water droplets, reflects off the back inside surface and then refracts again upon exiting. The spreading of the colors is due to dispersion.
Dispersion is the physical principle that allows the formation of a rainbow in the sky. Take note that the angle between incoming sunlight and outgoing sunlight is approximately .
Please follow this link to a nice graphic of the rainbow with a larger perspective: Rainbow Graphic.
You can see that the 42 degree angle is always formed between the viewer's line of sight and the path of the sunlight in the sky.