Materials for making very fine single or double slits for the study of diffraction and interference effects of light. A glass microscope slide is coated with a colloidal suspension of graphite. Single slits are scratched across dried graphite with a razor blade and a covering slide taped over the slits for protection. Separation of double slits is calculated by measuring the thickness of a single blade. Red and blue cellophane filters are provided to give light of restricted wavelength ranges. A showcase-type incandescent filament lamp is provided as a longline source. Instructions included.