What you’ll need
- A transparent, wide glass container (e.g., a gratin dish)
- Some milk
- 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
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.