Thomas Young observed interference of light and measured the wavelength of light in this classic experiment, performed around 1801. It provided the clinching evidence in Young's arguments for the wave model for light. This Demonstration shows two light waves of the same
wavelength (shown in red and blue for ease of viewing) which have passed through two narrow slits and, as a result of diffraction, are traveling at an angle
to their original direction of propagation. At the angle shown above, red and blue wave crests alternate and cancel, resulting in a dark spot on the screen. At other angles
, red and blue crests fall on top of one another and reinforce, resulting in a bright spot on the screen. The user can vary the angle
, the wavelength
, and the distance between the slits
to see how changes in the three variables affect the interference pattern formed by the two waves. The "double slit" formula
, the number of wavelengths difference in the path length of the two waves.