The wavelength of light is thus expressed as represents the wavelength in a vacuum. The phase difference between two waves can change if they pass through different materials with different indexes of refraction. The light leaving these slits flares out or diffracts, and interference occurs in the region beyond the screen. A fringe pattern forms on a viewing screen. This behavior was visible in other types of waves, thus proving light was a wave: the path length difference determines the appearance of each point in the viewing screen when a young"s double-slit interference experiment is performed. The path length difference is expressed as: the conditions for maximum and minimum intensity are expressed by: represents the angle the light path makes with a central axis. represents the slit separation. 35-3 interference and double-slit intensity: coherence in light (and therefore perceptible interference) is extant when the phase difference remains constant when two light waves meet at a point.