Theory behind this practical:

A single ray of light from a light source is an excellent way of seeing how waves behave. You will be able to see and measure that angles of incidence and reflection when a ray hits a mirrored surface and to see refraction when a ray passes from one medium to another of a differing optical density.


Reflection – set up the apparatus as seen in the photographs below. Make dots in pencil along the line of the rays and draw along the back surface of the mirror. Now join the dots to draw the rays. Construct the “normal” (a line at 90° to the mirror) then measure both the angles of incidence and reflection. Repeat for a variety of angles.

Note – you should find that the angle of incidence always equals the angle of reflection. If you are out by a couple of degrees, it is probably due to the sharpness of your pencil!

Refraction – Use the same light source and shine it into the side of a Perspex block. Draw around the block and, as above, draw pencil dots inside the light rays then join them up after.

Note that as the light travels from a low density medium (air) into a higher density medium (Perspex), the light slows down and bends towards the normal (red lines at 90° to the edges of the clock) and when it leaves the block, it speeds up again and bends away from the normal. Also, as the sides of the block are parallel, the ray arriving at the block and the one leaving are parallel with each other (if continued). You can try this again with blocks of different materials and the severity of the bending will change.

Technician notes:

  • Power packs
  • Lightboxes
  • Combs – must have single slit option
  • Perspex rectangle blocks
  • Mirrors
  • Protractors and rulers
  • Variety of other blocks for investigation


Usual lab rules must be adhered to:

  • Bags away and stools tucked under
  • Metal lightboxes get hot, do not handle when warm – minimise the time that they are on
  • Notify the teacher immediately if any breakages occur

For further/more detailed information, check all reagents and practices at CLEAPSS.