Double-slit experiment. Interference patterns observed in a modern-day replica of Thomas Young's double-slit experiment. A coherent beam from either a red Helium-Neon laser (632.8 nm) (top panel), or a green Helium-Neon laser (543.4 nm) (bottom panel), is used to illuminate two 25-micron-wide, 75-micron-spaced slits (double-slit). The resulting diffraction patterns are observed at the same distance from the double-slit. Note the fringe spacing is smaller for shorter (green) wavelength. This is a classic demonstration of the wave nature of the light.