DIFFRACTION GRATING
The diffraction grating has narrow slits very close together. If a coherent light source like a laser is used, with one wavlength then an interference pattern will appear on a screen. The maximas will be at an angle where the is a multiple number of wavelengths difference. From the diagram when d sin( e ) = n l, where l is the wavelength, d is the slit separation and n is the order of the maxima.