A ray of light travels from water into air and hits the boundary at an angle to the normal. Explain what happens to the ray as it crosses the boundary, including what happens to its speed, direction, and wavelength.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The light ray speeds up when it leaves water and enters air because the refractive index of air is lower than that of water. As a result the wavelength increases while the frequency remains unchanged. The increase in speed and wavelength causes the ray to bend away from the normal – this change of direction is refraction.
Examiner tips
- Use the word "refracts" and state the direction change (away from the normal).
- Mention the speed increase and wavelength increase explicitly.
- Remember that frequency is unchanged – this is a key point.
Common mistakes
- Saying the ray bends toward the normal, or confusing the direction of bending.
- Forgetting that the frequency does not change.
Mark scheme (5 marks)
- The ray speeds up as it crosses from water into air (because air is less optically dense than water)
- The ray bends/refracts away from the normal
- The frequency of the light does not change
- The wavelength increases as the ray enters air
- The change in speed (and wavelength) causes the change in direction / this is the process of refraction
Key terms in this question
Related
- All Eduqas A-Level Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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