The instrument that allows one to actually measure the wavelength of light is a diffraction grating, which is a transparent or reflective optical plate ruled with fine parallel and evenly-spaced grooves. The incoming light wave is spread out as it passes between the grooves and light emerging in specific directions, which depend on the wavelength, is reinforced by interference. Thus, gratings can separate a beam of light into its component colors the way prisms do. laser.physics.sunysb.edu/~wise/wi... spot.pcc.edu/~aodman/physics%2012... Goodbye
