Reflection, Refraction and Total Internal Reflection

When a wave meets a boundary between two different media, it can be reflected, transmitted (refracted), or both. The behaviour of light at boundaries underpins optical fibres, lenses, prisms, and a vast range of technologies. This lesson covers the laws governing these phenomena and the conditions required for total internal reflection.

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Reflection

When a wave hits a surface and bounces back, it is reflected. The law of reflection states:

• The angle of incidence equals the angle of reflection: θᵢ = θᵣ
• The incident ray, the reflected ray, and the normal to the surface all lie in the same plane

Angles are always measured from the normal — the line perpendicular to the surface at the point of incidence — not from the surface itself. This is a common source of error.

Reflection occurs for all types of wave. Light reflecting from a mirror, sound echoing from a cliff face, and water waves bouncing off a harbour wall all obey the same law.

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Refraction