In reality light is light. In photography you can distiguish between visible light and wavelengths we can't perceive. However in Cycles the users can exploit the fact that each ray can be isolated depending on the last type of material it touched. While treating rays differently when they transmitt a glass object than those that bounced off a reflective surface is not realistic, it offers a whole range of possibilities from subtle enhancements to surrealistic effects like particles and VFX. This is done using the light path node which I will discuss in this presentation.