Refraction of light — Physics, 14–17 years
Light changes direction when it crosses into a material where it travels at a different speed. This explains bent-looking objects, lenses and many optical instruments.
Light changes course
When a ray enters glass or water at an angle, one side of the wavefront slows before the other side. The ray therefore turns, rather like a shopping trolley whose one wheel reaches thick carpet first. Entering a denser optical material usually bends the ray towards the normal line.
Why explain the bend?
A straight-line model of light could not explain why a coin appears to move when water is poured into a bowl, or how a lens makes an image. Refraction gives a reason: light travels at different speeds in different materials. Once that is understood, the bending can be predicted rather than memorised.
A ray entering glass
A ray enters glass from air at 30° to the normal. Take the refractive index of glass as 1.5 and air as 1. Snell’s law gives 1 × sin 30° = 1.5 × sin r, so sin r = 0.5 ÷ 1.5 = 0.333. Therefore r is about 19.5°, closer to the normal.
The angle trap
Many people measure the angle from the surface instead of from the normal. This is reasonable because the surface is visible and easy to point at, while the normal is an imagined perpendicular line. In refraction calculations, both incident and refracted angles must be measured from that normal.
Optical devices
Refraction is used in spectacles to redirect light so that the retina receives a sharp image. Cameras, microscopes and binoculars also use shaped pieces of glass or plastic to bend rays in controlled ways. The effect is useful here because the material and shape can be chosen to produce a particular image.
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