Explain what happens when a wave travelling through one medium reaches the boundary with a different medium.

Eduqas GCSE Physics — 5.2 Waves at material interfaces · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

When light travels through glass and reaches the boundary with air, some of the light passes through and some does not.

Model answer (4 marks)

When a wave reaches a boundary between two media, part of the wave is reflected back into the first medium and part is transmitted into the second medium.
The transmitted part changes direction – it is refracted – as it crosses the boundary.
This change in direction occurs because the wave’s speed changes when it enters the new medium.

Examiner tips

Use the word ‘reflected’ and ‘transmitted’ to show you understand the two parts of the wave.
Explain that refraction is due to a change in speed, not just a change in direction.
Keep the answer concise – 4 marks can be earned with a short paragraph.
Mention the boundary explicitly to show context.

Common mistakes

Confusing reflection with refraction, or saying the wave is only reflected.
Failing to mention that the transmitted wave changes direction.
Using vague terms like ‘bounces’ instead of ‘reflected’ or ‘bends’ instead of ‘refracted’.

Mark scheme (4 marks)

Some of the wave is reflected at the boundary
Some of the wave is transmitted through the boundary into the new medium
The transmitted wave changes direction / is refracted as it crosses the boundary
The change in direction occurs because the wave changes speed in the new medium

Key terms in this question

boundary · medium