State the laws of refraction. Draw a labelled diagram to show the refraction of light through a rectangular glass slab, and explain why the emergent ray is parallel to the incident ray.

1. 1 · Define in one line. Refraction is the bending of light when it passes obliquely from one transparent medium to another, because its speed changes.
2. 2 · Write the two laws.
   • The incident ray, the refracted ray and the normal at the point of incidence lie in the same plane.
   • Snell's law: sin i / sin r = constant (the refractive index n₂₁) for a given pair of media and colour of light.
3. 3 · Draw the slab diagram. [Diagram: slab ABCD; incident ray to O on AB; dotted normals at O and O′; refracted ray OO′ bent towards the normal; emergent ray from O′ parallel to the dotted extension of the incident ray; ∠i, ∠r, ∠e and the lateral displacement marked]
4. 4 · Explain the parallel ray. At AB the ray bends towards the normal (air to glass). At CD it bends away by the same amount (glass to air). The faces are parallel, so ∠e = ∠i and the ray comes out parallel, only shifted sideways.
5. 5 · Link to daily life. A line seen at a slant through a thick glass tabletop looks shifted sideways, but not tilted: this is the same lateral displacement.
6. 6 · Close in one line. So a glass slab displaces a ray sideways without changing its direction.

Scoring tip: Draw normals as dotted lines at both faces and keep ∠e visibly equal to ∠i. Label all three rays.