Double-slit diffraction. Modern-day replica of Thomas Young's double-slit experiment. Here a coherent beam from a diode laser (635 nm) is incident on two closely-spaced slits (25 micron slit width, 75 micron distance between slits). Light transmitted by the two slits diffracts and the resulting interference pattern is observed on a screen about 20 cm away. Insert shows a close-up of the interference pattern, taken in a separate shot. This is a classic demonstration of the wave nature of the light.