Thomas Young's double-slit experiment demonstrated that light has wave-like properties by showing that light passing through two slits will produce an interference pattern of bright and dark fringes on a screen. The experiment found that the distance between fringes is determined by the wavelength of light and the distance between the slits and screen. Increasing the wavelength or distances between slits or screen decreases the fringe spacing. This relationship can be used to calculate unknown wavelengths from measured distances. For example, in an experiment using sodium light with 0.3mm slits 1.2m from a screen showing 2.4mm spaced fringes, the wavelength is calculated to be 589nm.