Double-slit experiment, Photo 2 of 3. Interference pattern observed in a modern-day replica of Thomas Young's double-slit experiment. A coherent beam from a Helium-Neon laser is incident on two closely-spaced 25-micron-wide slits and the resulting diffraction pattern is observed at some distance away. In this series the camera to the double-slit distance is fixed. Photo 1 and Photo 2 use two different Helium-Neon lasers, red (632.8 nm) and green (543.4 nm), illuminating the same double-slit (75 micron slit separation). Note the fringe spacing is smaller for shorter (green) wavelength. Photo 3 uses the same red laser as Photo 1, but the separation between the individual slits is smaller (58 micron). Note the increased fringe spacing in Photo 3 compared to Photo 1. This is a classic demonstration of the wave nature of the light.