> What's a diffraction grating

A set of lines separated by not much: I once saw a piece of glass ruled with 3000 lines per inch diffract a beam of monochromatic light by thirty degrees or so, much to the teacher's surprise.

> and where did you find it?

If you can't get to a physics lab or a school, turn over a CD or DVD under white light, and look for the reflection with the rainbow pattern. The effect's wavelength-dependent.

.... Oh, and look up "double-slit experiment" for why I made the comment in the first place. We now return you to the KISS Principle.