Core practical: refraction in a glass block

1. Put a rectangular glass block on paper and draw round it.
2. Shine a narrow ray from a ray box (with a slit) at the block at an angle.
3. Mark the incoming ray and the ray coming out with small crosses, far apart. Remove the block and join the crosses through the outline.
4. Draw the normal (at 90° to the surface) where the ray enters and leaves.
5. Measure the angle of incidence and angle of refraction with a protractor, from the normal.
6. Repeat for different angles of incidence. Work in a darkened room so the ray is easy to see.

Entering the glass, light slows down and bends towards the normal: the angle of refraction is smaller than the angle of incidence.
Leaving the glass, it speeds up and bends away from the normal.
The ray that comes out is parallel to the ray that went in, but shifted sideways.
A ray along the normal (0°) passes straight through without bending, though it still changes speed.

Light travels at different speeds in different materials: about 3 × 10⁸ m/s in a vacuum and air, and slower in glass.
When a wavefront hits the boundary at an angle, one side slows down first, so the wave changes direction.
Sources of error: a thick ray makes it hard to mark the centre; measure angles carefully from the normal, not from the surface.