Reflection and Refraction at a Boundary — Reflection & Refraction | IGCSE Physics

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 9% of your exam marks.

stable

Medium

Stable9%

Ray diagrams, Snell's Law and critical angle calculations appear regularly.

Reflection is what happens when a wave hits the boundary between two media and bounces back into the original medium without crossing the boundary
Refraction is what happens when a wave passes through a boundary between two transparent media and changes direction as it does so
The word medium (plural media) means any material that transmits the wave, such as air, glass, water, or perspex

Every ray diagram for reflection or refraction is measured from an imaginary line drawn at 90° to the surface at the exact point the ray hits; this line is called the normal
Angles are measured between the ray and the normal, not between the ray and the surface itself

angle of incidence (i) = angle of reflection (r)

where:
- angle of incidence is the angle between the incoming (incident) ray and the normal
- angle of reflection is the angle between the outgoing (reflected) ray and the normal
On a flat mirror, every ray reflects neatly, which is specular reflection and produces a clear image. On a rough surface every tiny patch tilts a different way, so reflected rays scatter, which is diffuse reflection and produces no image

When a light ray crosses from one transparent medium to another, its speed changes, and that speed change forces the ray to change direction
The bend depends on which way the speed is changing:
- Entering an optically denser medium (for example, air into glass): the ray slows and tilts towards the normal
- Leaving a denser medium for a thinner one (for example, glass back into air): the ray speeds up and tilts away from the normal
A ray that strikes the surface along the normal (perpendicular, 0°) keeps going straight; only its speed changes, never its direction

During refraction, two wave properties change and one stays the same:
- Speed changes (slows in denser medium, speeds up in less dense one)
- Wavelength changes in step with the speed (using v = f × λ)
- Frequency stays the same, because the source still emits the same number of crests per second, so the same number must arrive in the new medium. A red ray of light staying red when it dives into water is the everyday evidence of this