History: 1900 BC early mirrors 1621 AD Willebrord Snell's law of refraction states N1sin(1) = N2 sin(2) 1650 Francesco Grimaldi observes bands of light within the shadow of a rod illuminated by a small source. 1665 Isaac Newton shows that white light is composed of many colors using a prism. 1801-1803 Thomas Young's wave theory of light. 1919 20 ft stellar interferometer built by Michelson measured diameter of Betelgeuse in 1920 at 0.047 arc second. The human eye sees 1 arc minute, a 6 inch scope sees 1 arc second. The largest moons of Jupiter appear 1 arc second in diameter from earth, Sirius 0.007 arc second. The Twin Keck 85 meter baseline can now measure 0.003 arc second. The CHARA array on Mount Wilson has 6 scopes with a 350 meter maximum baseline, and can measure 0.005 arc second. The VLTI in Chile has a 200 meter baseline capable of 0.001 arc sec.