22 Light Wave fronts rays Reflection Refraction Total

22 Light • • Wave fronts & rays Reflection & Refraction Total internal reflection Dispersion • 12, 19, 24, 36, 39, 49, 64. 1

Rays & Wave Fronts • ray = ____ of light • wave front: overall ______ of a wave at an instant of time • rays are ______ to wave fronts. 2

Diverging and Non-Diverging Rays 3

Law of Reflection θr = θi 4

Refraction • Light speed _____ in media, e. g. , _______ in water than in air. • Index n of medium = c/v • e. g. n(air) =1. 0, n(water) = 1. 33. • Snell’s Law: • _____________ 5

v = fl in medium • In a medium, the _______ of light is unchanged but the ______ changes • l(medium) = l(air)/n • Ex. l(air) = 650 nm enters water index n = 1. 33. • l(water) = l(air)/n = _________ 6

Total Internal Reflection • All light in a medium n 1(incident) > n 2(ambient) is reflected and none is refracted. • Incident angle > θ 1 c in: n 1 sin θ 1 c = n 2 sin 90° = n 2 7

Dispersion • Index depends on wavelength • Larger for shorter wavelengths • Refraction angles vary 8

Summary • Rays perpendicular to wave fronts • Refraction determined by indexes • total internal reflection may occur when light is in material with index > surroundings • different color light has different index in same material 9