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InkFlaskCambridge IGCSE

Light · Physics

Glass blocks and prism

What it is for

Refracts (bends) rays: rectangular and semicircular blocks, a prism.

Mistake to avoid Moving the block while marking the rays: keep it on its drawn outline until every ray position is marked.

Drawn in ink the way exam diagrams show it; the card in the list shows the lab kit's model.

How it is asked

  • Describe the experiment: draw round the block, shine a ray at it, mark the ray in and out with crosses, then remove the block and join the points.
  • Draw the normal and measure the angles of incidence and refraction from it, not from the surface.
  • State the result for a rectangular block: the ray bends towards the normal going in and away coming out, and leaves parallel to the ray that went in.

Exam-style question

A ray of light meets one side of a rectangular glass block at an angle of incidence of 40°.

  1. Is the angle of refraction in the glass larger or smaller than 40°? Explain.
  2. How does the ray leave the opposite side of the block?
  3. Why does a ray that meets the block along the normal not bend?
Show what a good answer says
  1. Smaller: light slows down as it enters glass, so it bends towards the normal.
  2. It bends away from the normal and leaves parallel to the ray that went in, shifted sideways.
  3. Its angle of incidence is 0°: it slows down in the glass but does not change direction.

Teaching points for checking your own answer, not official Cambridge marks.

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