The normal is an imaginary line at 90° to a surface, drawn where the ray hits it. Every angle in a ray diagram is measured from the normal, never from the surface.

At a mirror, the angle of reflection equals the angle of incidence.

1. A ray makes 25° with the surface of a mirror
2. angle of incidence = 90° − 25° = 65°
3. angle of reflection = 65°
4. the incident and reflected rays are 65° + 65° = 130° apart

The classic slip: using the angle to the mirror surface. Draw the normal first, then measure.

• Specular reflection: a smooth surface (a mirror, still water) reflects parallel rays in a single direction, so you see a clear image.
• Diffuse reflection: a rough surface (paper, a matt wall) scatters parallel rays in many directions. You see the surface, but no image.

Each tiny part of a rough surface still obeys the law of reflection: the normals just point in different directions.

You met refraction in the waves unit: light changes speed, and so direction, as it crosses into a new material. This practical measures it.

1. Put the block on paper and draw round it
2. Draw a normal at 90° to one side with a protractor
3. Aim a narrow ray from a ray box at the normal, at a measured angle
4. Mark the ray in and out with crosses, then remove the block
5. Join entry and exit points; measure the angle of refraction from the normal
6. Repeat for several angles of incidence

Independent variable: the angle of incidence (or the material). Dependent: the angle of refraction. Control: the same block, the same narrow ray.

Going into glass the ray bends towards the normal; coming out it bends away. Along the normal it does not bend at all.