Explain what happens to a beam of monochromatic light as it passes from air into a glass block and then back out into air on the other side, including what happens to its speed, wavelength, and direction at each boundary.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
When the beam enters the glass the speed of light decreases from c to c/n, where n>1 for glass. Consequently the wavelength shortens from λ_air to λ_glass=λ_air/n. The change in speed causes the ray to bend towards the normal – the angle of refraction is smaller than the angle of incidence.
When the beam leaves the glass back into air the speed increases again to c and the wavelength returns to λ_air. The ray bends away from the normal, so the angle of refraction is larger than the angle of incidence. If the two surfaces are parallel the two refractions cancel and the emergent ray is parallel to the incident ray, although it is displaced laterally.
When the beam leaves the glass back into air the speed increases again to c and the wavelength returns to λ_air. The ray bends away from the normal, so the angle of refraction is larger than the angle of incidence. If the two surfaces are parallel the two refractions cancel and the emergent ray is parallel to the incident ray, although it is displaced laterally.
Examiner tips
- Use the word ‘bends’ and specify direction (towards/away from normal).
- Show the relationship between speed, wavelength and refractive index.
- Mention that the emergent ray is parallel to the incident ray for parallel surfaces.
Common mistakes
- Confusing the direction of bending (e.g., saying it bends away when entering glass).
- Forgetting that speed and wavelength return to their original values on exiting.
Mark scheme (5 marks)
- The light slows down as it enters the glass (from air into glass).
- The wavelength decreases as the light enters the glass.
- The light bends towards the normal as it enters the glass (because it is entering a denser medium).
- As the light exits the glass back into air, its speed and wavelength increase back to their original values.
- The light bends away from the normal as it exits the glass into air (less dense medium), and if the surfaces are parallel the ray emerges travelling in the same direction as the incident ray (though displaced).
Key terms in this question
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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