Explain what happens to the speed, frequency, and wavelength of a wave when it undergoes refraction as it moves from a less optically dense medium into a more optically dense medium.

Cambridge International IGCSE Physics (0625) — 3.1 General properties of waves · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

When a light wave passes from a less optically dense medium into a more optically dense medium the
1. speed decreases;
2. frequency remains unchanged;
3. wavelength shortens;
4. the shortening of λ follows from v=fλ, so with v↓ and f=const, λ must ↓.

Examiner tips

Use the word ‘decreases’ for speed and wavelength; ‘remains unchanged’ for frequency.
Show the relationship v=fλ to justify the change in wavelength.
Keep the answer concise – 4 points only.
Use UK spelling (e.g. ‘optically’).

Common mistakes

Saying the frequency changes; it stays constant.
Confusing ‘optically dense’ with ‘high refractive index’ without linking to speed.
Using the wrong direction for wavelength change (claiming it increases).

Mark scheme (4 marks)

The speed of the wave decreases (when entering a more optically dense medium).
The frequency of the wave remains unchanged / constant.
The wavelength of the wave decreases.
The wavelength decreases because speed decreases but frequency stays constant, linked by v = fλ (or equivalent correct reasoning).

Key terms in this question

refraction · speed · frequency · wavelength · optically dense medium