The double-slit experiment demonstrates that two point sources of light can create an interference pattern. A single light source is passed through two narrow slits instead of actually using two lamps—this method ensures that the two waves diffracting out of the slits are in phase. The waves create an interference pattern on the screen like this:
The interference pattern is a series of nodal lines and antinodes. The nodal lines are evenly spaced by on a screen separated from the slits by a distance of . If is the distance between the slits and is the wavelength of the light, then
where all the variables are measured in metres (m). One consequence of this is that, all other things being equal, a shorter wavelength will result in a smaller value of .
Unless you use a colour filter, all the colours of the rainbow present in white light will go through the slits. Red light has a large value of and violet light has a small value. This means that the nodal lines of different colours will be at different locations on the screen. For example, if the nodal lines for violet and green happen to overlap at one point, then you will see all the colours except violet and green at that point (since that area is an antinode for the other colours).