A JavaSim simulation demonstrating how light is refracted and reflected by raindrops to form the optical phenomenon we all know and love: the rainbow.
When sunlight passes through raindrops, the total angle change after refracting, reflecting, and dispersing stops at a certain limit dependent on the wavelength of light.
For each ray of sunlight, it strikes the raindrop with an angle of incidence, 
According to Snell's law, when changing mediums from air to water or water to air, an angle of refraction 
where 
Solving the geometry of the problem, we can see that the total deflection angle, 
or, solving for
For a certain refractive index
This angle where 
The current output of the simulation:
A zoomed in version of the bottom 6.5% of the outputted rays is visible in the bottom right.
RaindropRainbow is a JavaSim simulation. After installing JavaSim, place RaindropRainbow.jar into the JavaSim folder (located in your Documents folder by default).