Reflection & RefractionEdexcel IGCSE Physics: Revision notes

Light waves are transverse waves — they can be reflected and refracted, just like all other waves.

When light hits a flat, smooth surface such as a mirror, it bounces off. This is reflection. Reflection always obeys the law of reflection:

angle of incidence = angle of reflection

Both angles are measured from the normal — an imaginary line drawn at 90° to the surface at the point where the light ray strikes it.

• The incident ray is the ray travelling towards the surface
• The reflected ray is the ray travelling away from the surface after reflection

Refraction is the change in direction (and speed) of a wave as it passes from one medium into another of different density, caused by a change in the wave's speed.

• Light travels more slowly in a denser medium (e.g. glass or water) than in a less dense medium (e.g. air)
• When light slows down entering a denser medium, it bends towards the normal
• When light speeds up leaving a denser medium, it bends away from the normal
• Light travelling exactly along the normal does not change direction, only speed

The refractive index ($n$) of a material measures how much it slows down and bends light. It is calculated using the angle of incidence ($i$) and angle of refraction ($r$) as light passes from air into the material:

$n = \dfrac{\sin i}{\sin r}$

A higher refractive index means the material bends light more and slows it down more. This can be investigated practically using a glass block: measuring the angle of incidence and angle of refraction for several rays and calculating $n$ for the glass.

When light travels from a denser medium into a less dense medium (e.g. glass to air), it bends away from the normal. As the angle of incidence increases, the angle of refraction gets closer to 90°.

At a specific angle of incidence — the critical angle ($c$) — the refracted ray travels exactly along the boundary (at 90°). Beyond the critical angle, no light is refracted out at all; instead all the light is reflected back inside the denser medium. This is total internal reflection (TIR).

The critical angle is related to refractive index by:

$\sin c = \dfrac{1}{n}$

Uses of total internal reflection:

• Optical fibres use repeated total internal reflection to transmit light (carrying information) along their length with very little loss
• Prisms use total internal reflection to redirect light through 90° or 180°, e.g. in periscopes and binoculars