To investigate the reflection of light by a plane mirror.
Set up a ray box, slit and lens so that a narrow ray of light is produced.
Place a 30 centimetre (cm) ruler near the middle of a piece of plain A3 paper. Draw a straight line parallel to its longer sides. Use a protractor to draw a second line at right angles to this line. Label this line with an 'N' for 'normal'.
Use the ray box to shine a ray of light at the point where the normal meets the mirror. This is the incident ray.
The angle between the normal and the incident ray is called the angle of incidence. Move the ray box or paper to change the angle of incidence. The aim is to see a clear ray reflected from the surface of the mirror.