[Physics] Calculating the rainbow angle

geometric-opticsoptics

I've read that the angle for viewing a Primary Rainbow is between 40-42 degrees. How would I go about showing this? How would I calculate that angle from scratch? I figured that Snell's Law and the Law of Reflection would be used for light bouncing off and through a (spherical) water droplet. But I only have a vague idea of how to begin.

I would imagine that once you know how to calculate the Primary Rainbow angle, calculating the Secondary Rainbow angle would be very similar if not identical process. Is this right?

Best Answer

Actually this is a bit of a tricky problem that only was solved relatively recently. The primary rainbow is generated by a beam that refracts once inside the drop, reflects off the back, then refracts again upon exit.

angles

You have to use snell's law and then some calculus to find the minimum scattering angle for the angle that produces the brightest light as illustrated below from ray tracing:

rays

For a fully worked example try here. And this site has some nice charts of the primary and secondary bows.