The Edge Of The Rainbow
Ordinary light is made up of light of many different colors. You can see these colors if you bounce the light of a thin film on a highly reflective surface. Oil slicks show this effect, where you see a whole spectrum of colors that result from the light reflecting off the surface of the oil and whatever is underneath. Here, a thin polymer film is receding across a highly reflective silicon substrate, leaving behind small droplets in its wake. The different colors tell you the thickness of the film, and, as you can see, it changes as you go across the film. The droplets form because the polymer doesn't like the surface and tries to bead up, just like water on a nonstick pan.
Image credit: ©VISUAL, Materials Research Science and Engineering Center, University of Massachusetts, Amherst