With a labelled diagram, show the path of a ray of light through a rectangular glass slab. Mark the angles and state what you observe.

1. 1 · Define refraction in one line. Refraction is the bending of light as it passes obliquely from one medium into another.
2. 2 · Draw the slab and the incident ray. Draw the slab outline, an incident ray meeting the top face at O and a dotted normal at O.
3. 3 · Draw the refracted and emergent rays. Inside the glass the ray bends towards the normal (∠r < ∠i). At the far face it bends away from the normal and comes out.
4. 4 · Label the diagram. [Sketch: incident ray, refracted ray, emergent ray, two normals; mark i, r and e, and the sideways shift]
5. 5 · State the observation. ∠e = ∠i, so the emergent ray is parallel to the incident ray but shifted sideways (lateral displacement).
6. 6 · Close. So light bends towards the normal entering glass and away from it leaving glass, by equal amounts at parallel faces.

Scoring tip: Mark every angle from the normal and draw the emergent ray parallel to the incident ray. Examiners check that the two are parallel.