A student shines a ray of light from water into air. The ray hits the boundary between water and air at an angle. Describe what happens to the ray of light as it crosses this boundary, and explain why this happens.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The speed of light in water is approximately 2.25 × 10⁸ m/s, while the speed of light in air is approximately 3.00 × 10⁸ m/s.
Model answer (5 marks)
The ray of light changes direction (refracts) as it crosses the boundary.
The ray bends away from the normal – the angle of refraction is greater than the angle of incidence.
The speed of light increases as it travels from water (2.25×10⁸ m s⁻¹) into air (3.00×10⁸ m s⁻¹).
Refraction occurs because light travels at different speeds in different media.
The wavelength of the light increases (but the frequency remains unchanged).
The ray bends away from the normal – the angle of refraction is greater than the angle of incidence.
The speed of light increases as it travels from water (2.25×10⁸ m s⁻¹) into air (3.00×10⁸ m s⁻¹).
Refraction occurs because light travels at different speeds in different media.
The wavelength of the light increases (but the frequency remains unchanged).
Examiner tips
- Use the word "refracts" and mention the normal. State the direction of bending (away from the normal). Give the numerical speeds to justify the speed increase. Explain that the change in speed causes refraction. Remember that frequency is constant, only wavelength changes.
Common mistakes
- Saying the ray bends towards the normal (wrong direction). Forgetting to mention that the frequency stays the same. Using vague terms like "changes speed" without specifying increase. Not linking the change in speed to the cause of refraction.
Mark scheme (5 marks)
- The ray of light changes direction (refracts) as it crosses the boundary
- The ray bends away from the normal (the angle of refraction is greater than the angle of incidence)
- The speed of light increases as it travels from water into air
- Refraction occurs because light travels at different speeds in different materials / media
- The wavelength of light changes (increases) but the frequency remains the same
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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