Why is it brighter under a rainbow? part 2

 

The next bit to consider is called the deviation of the ray. One red ray is going into the water droplet top left heading down and right. It emerges after refracting twice and reflecting off the back of the droplet to head down and left. It has been turned but by how many degrees has it been rotated? This is the deviation. The red dotted line at the top shows the original direction. The ray is pulled out of line by (a-b) at this point. It is then bent back on itself. The internal angle is 2b but the rotation is 180 degrees less 2b. Finally there is a bending by another (a-b) coming out. This adds up to a total deviation D of (a-b) + (180-2b) + (a-b). The derivation of this is critical to the analysis I'm following from this site and was one of the bits I didn't really understand.