Critical Angle


  • A rectangular aquarium
  • A few drops of milk (or some powdered milk)
  • A laser or mini Maglite flashlight focused to make a beam
  • A darkened room


  1. Fill the aquarium with water.
  2. Shine your light through the aquarium
  3. Add in milk one drop at a time (or for powdered milk, one pinch at a time) until you can clearly see a light beam in the water. Stir after each drop.
  4. Once you can see your beam of light, angle your laser/flashlight up towards the surface of the water.


If you are lucky enough, you should be able to see both the reflected and the refracted beam of light. Water is actually more reflective than mirrors, so much of the light bounces off of the water’s surface. You should also notice that the light reflects at the same angle that it hits the water. This is true in all cases. When the angle is small enough, though, some light will leave the water and be refracted.

In other activities, we have talked about refraction being the bending of light. However, we haven’t talked about the angle at which it bends. The angle that light moves through a substance is called the critical angle. It is the greatest angle light can move at without any light being lost. You should notice that the critical angle of the light as it moves through the water is different from when it moves through the air. When light goes from a substance that is hard to move through (water) to a substance that is easy to move through (air), it refracts away from the substance.