Fun with Diffraction Gratings!! Preprinted Rainbow Peepholes. Always in stock and ready to ship. The ideal teacher incentive. Great in the classroom when a quality diffraction grating is needed for the study of Light and Color. Just look through your Rainbow Peephole at any light source...the rainbow starbursts will amaze you! RAINBOW PEEPHOLES: 13,500 Line/Inch Diffraction Grating - 1 11/16" Diameter, 1/2" Window - Heavy Cardboard.