This experiment was very important in demonstrating the wave nature of light. It is very simple, consisting only of a light source and two screens, one with two slits. Light passes through each slit, and the light that passes through each slit interferes with the light that passes through the other and forms an interference pattern on the screen at the right. Constructive interference occurs so that a bright fringe is formed on the screen when the path difference between light from the two sources is a whole number of wavelengths.
The experimental layout is illustrated below where the distance
to the first bright fringe on the screen is shown. Since it is the first bright fringe the path difference between light from the two slits must be equal to one wavelength,
Because
is an angle in both triangles we may say approximately
In generalis the distance between successive fringes, the fringe spacing.