How Rainbows Form 

Copyright 1999 Rebecca McDowell.
1.  Light from Sun strikes raindrop  
2.  Some of the light is reflected  
3.  The rest of the light is refracted  
4.  Light splits into component colours  
5.  Reflected at rear of raindrop (TIR)  
6.  Refracted again as it leaves raindrop 
7.  Colours are further dispersed 

Click on the links above for more detail on ech step.