Originally Posted by PR_Phil

when the medium through which light travels changes, the phase velocity of the wave changes but the frequency remains the same, the angle on incidence, (the angle at which the light hits the surface of the medium) is equal to the angle of refraction (the angle at which the light leaves the surface of the new medium. Common plastics used in the creation of rhinestones for dresses are injection molded at high temperature and the surfaces retract during the cooling phase when they come out of the mold and shrink. The concave surface causes the refracted light to converge into a focal point after which it begins to disperse again, the models arms where close to the focal point, causing bright light spots.

I could see you saying "your knowledge of physics sucks" but to say he doesn't know much about photography is probably pushing it a bit, I think photographers using semi translucent material to refract and distort light in a manner that creates spots on skin that look like arm syphilis is probably a pretty advanced technique.