Double Slit Interference

One of the first experiments to demonstrate the wave nature of light was carried out by Thomas Young in 1800. This experiment involved illuminating two small slits with light from a single source. In the drawing below, the light bulb and slit with single hole represent a coherent light source (yellow lines). In coherent light all waves have the same phase and wavelength. This light is incident on two small slits. Both the slit size and the distance between the two slits are the same order of magnitude as the wavelength of light. Each of the two slits acts as a source of light waves (red and green lines) - each of the slits is a new "point source," light expands spherically from point sources.

At any given point to the right of the double slits, light waves originating from each of the slits will have to travel a given distance from the center of the slit. A different distance of travel implies a different relative phase between the two light waves. When the relative phase is a multiple of the wavelength, constructive interference will occur. Along the dashed lines in the drawing, constructive interference will take place. When the relative phase is a multiple of the wavelength plus half the wavelength, destructive interference occurs.