What is a diffraction grating? When to use it?

Read these

http://en.wikipedia.org/wiki/Diffraction

http://physics-animations.com/Physic...sh/DG10/DG.htm

Diffraction Grating Handbook
http://gratings.newport.com/library/...k/handbook.asp

in above book
chapter 13. APPLICATIONS OF DIFFRACTION GRATINGS

Diffraction grating interactive java animation
http://www.physics.uq.edu.au/people/...g/grating.html

Good set of diffraction interactive animations
http://www.falstad.com/wave2d/