Refraction of Light at Plane Surfaces
Light bends at the boundary between two media because its speed changes; this is refraction. The paper tests Snell's law, refractive index, total internal reflection and everyday effects such as a pencil looking bent in water.
Laws and definitions
- The incident ray, the refracted ray and the normal at the point of incidence all lie in the same plane.
- Snell's law: sin i / sin r = n21, a constant for a given pair of media, where n21 = n2/n1 = v1/v2.
- Absolute refractive index n = c/v, with c = 3 × 10^8 m/s. Air is about 1, water 1.33, glass 1.5, diamond 2.42.
- From rarer to denser medium the ray bends towards the normal, so r is less than i; from denser to rarer it bends away.
- A ray falling normally (i = 0) passes straight through undeviated.
- Through a rectangular glass slab the emergent ray is parallel to the incident ray but laterally displaced.
Total internal reflection
It occurs only when light travels from a denser to a rarer medium and the angle of incidence exceeds the critical angle C, where sin C = 1/n. For glass with n = 1.5, sin C = 0.667, so C is about 42°. Applications include optical fibres, endoscopes, totally reflecting prisms and the sparkle of a cut diamond.
Worked example
Light passes from air into glass (n = 1.5) at i = 30°. Then sin r = sin 30°/1.5 = 0.5/1.5 = 0.333, so r is about 19.5°. The ray bends towards the normal because glass is optically denser than air.
Traps
Angles are always measured from the normal, never from the surface; violet is deviated most because its refractive index is highest; apparent depth equals real depth divided by n, which is why a pond looks shallower than it is.