What you’ll need

  • A transparent, wide glass container (e.g., a gratin dish)
  • Water
  • Some milk
  • Teaspoon
  • A white-light flashlight (you can use the one on your smartphone)
  • A white surface or base (e.g., kitchen roll)
  • A space that can be darkened

How to do it

1. Fill the gratin dish with water and add a teaspoon of milk. Set it down on a white surface.

Read More

2. Dim the room and shine the flashlight from the side into the dish containing the milk water and observe it from above. You may need to hold the flashlight very close to the dish, depending on how strong the beam is.

Read More

3. Now, experiment with different angles. Use the flashlight to illuminate the dish from behind and position yourself so that the dish is at eye level and you’re looking directly at the light through the milk water.

Read More

Well spotted!

If you illuminate the dish from the side and observe the rays from above, the milk water looks white or blueish. But if you position the flashlight behind the dish so that you’re looking directly into the light source, the light source looks yellow or even orange-red, just like when the sun goes down!

What’s the secret?

White light is made up of different wavelengths, which we perceive as different colors. The light is scattered and reflected on the finely distributed fat and protein particles in the milk water – short-wave (blue) light most strongly, but long-wave (red) light hardly at all.

When the light shines from the side into the milk-water mixture and we look at it from above, we mainly see the scattered, bluish light. It’s different when we look straight through the dish into the light source: Since the blue light is scattered in all directions as it passes through the milk water, it’s mainly the longer-wave yellow and orange portions of the light that reach us. This experiment works best with the widest possible vessel, as the light then has to travel further through the milk water. The further the distance over which the blue portions of light are scattered, the less blue light reaches our eye and the more intensely red-dish the light source appears.

Incidentally, this is the same principle that causes a cloudless sky to appear blue because the light of the sun is scattered in the atmosphere. However, if the sun is low above the horizon, the light has to travel a much longer distance through the atmosphere than at noon, when the sun is right above us. Therefore, just before sunset, only the orange-red portion of sunlight reaches us.