Skip to content

Snell’s Law


To observe the critical angle when light moves from a denser to a less densec optical medium.

HOW DOES IT WORK?                                                                          

Time Allocation: 45 Min

Refraction is the bending of the path of a light wave as it passes across the boundary separating two media. Refraction is caused by the change in speed experienced by a wave when it changes medium. This relationship between the angles of incidence and the indices of refraction of the two media is known as Snell’s Law. Snell’s law applies to the refraction of light in any situation, regardless of what the two media are.

When the angle of incidence reaches a certain critical value, the refracted ray lies along the boundary, having an angle of refraction of 90-degrees. This angle of incidence is known as the critical angle; it is the largest angle of incidence for which refraction can still occur. For any angle of incidence greater than the critical angle, light will undergo total internal reflection.


  • Experiment Booklet
  • Digital Software
  • A rectangular glass block
  • A ray box
  • Protractor
  • A4 paper
  • Pencil and ruler


  1. Place the rectangular glass block on the sheet of paper and mark the position of the glass block.
  2. Set up the equipment so that the ray from the light box shines through the longer end of the rectangular block. As per Fig 1.
  3. Gradually increase the angle of incidence and observe what happens to the angle of refraction.
  4. Find the angle of incidence for which the refracted ray shines along the surface of the glass – in this position the refracted ray looks like it has disappeared as per Fig 3.
  5. Mark the position of the incident ray at the point of the laser (A), the point where the light ray enters the glass block (B), against the glass block where the light ray emerges from the block (C), as well as a point further from the glass block (D) as per Fig 3.
  6. Draw in the normal in the centre of the rectangular at point (C) as shown in Fig 3.
  7. Use the protractor to measure the critical angle (ic).
  8. Further increase the incident angle and observe what happens if the incidence angle exceeds the critical angle.