A student shines a ray of light through a rectangular glass block. The ray enters the glass from air, travels through the glass, and then exits back into the air on the other side. Describe what happens to the ray of light at each boundary, including what happens to its speed and direction.

Eduqas A-Level Physics — 2.6 Refraction of light · Describe · 5 marks · View as Markdown

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

Model answer (5 marks)

At the air–glass interface the ray slows down because the refractive index of glass is higher than that of air.
The ray bends towards the normal as it enters the glass.
At the glass–air interface the ray speeds up again as it returns to a medium with a lower refractive index.
The ray bends away from the normal on exiting.
The emerging ray is parallel to the incident ray but displaced laterally.

Examiner tips

  • Use the terms ‘slows down’, ‘bends towards the normal’, ‘speeds up’, ‘bends away from the normal’, and ‘parallel but displaced’ – these are the exact phrases the mark scheme rewards.
  • Show the sequence of events clearly – one sentence per boundary – to demonstrate understanding of refraction.
  • Remember to mention the change in speed at each boundary – this is a key point in the scheme.

Common mistakes

  • Confusing the direction of bending (e.g. saying it bends away from the normal when entering glass).
  • Omitting the change in speed or the fact that the emerging ray is displaced but parallel to the original.

Mark scheme (5 marks)

  1. At the air-to-glass boundary, the ray slows down
  2. At the air-to-glass boundary, the ray bends towards the normal
  3. At the glass-to-air boundary, the ray speeds up
  4. At the glass-to-air boundary, the ray bends away from the normal
  5. The ray emerging from the glass is parallel to (but displaced from) the original ray entering the glass

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