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Why sunsets are red experiment

sunset experiment

You will need:

  • 1 large clear container filled with 5 litres of water
  • 50 mL Milk
  • A strong source of light, eg. spotlight, projector lamp
  • A darkened room


  1. Pour about 50 mL of milk into the water. It is often best to use only a little at first, trying the following steps out and only adding more if the intended effect is not apparent.
  2. Turn the spotlight on and darken the room.
  3. Face the light towards the milky water.
  4. The water near the milk will look white, yet if you look on the other side of the container the colour should be a mixture of orange and red. Something has been happening to the light as it passed through milk solution!

What is going on?

The demonstration just performed is a classic way of showing how sunsets work.
White light is comprised of all of the colours of the rainbow i.e. the light spectrum.
The different colours you see represent the different wavelengths of light.
A orange object is only 'orange' when you see the orange wavelength of light being reflected off it.

When light travels through a substance, some of the visible light wavelengths are absorbed whilst the other light is reflected. As the light entered the milk solution, the light in the blue end of visible spectrum was scattered by the suspended milk solids. This left the lower energy wavelengths of orange and red to pass through the solution, creating the orange/red colour seen in the experiment.

So how does this relate to sunsets?

The daytime sky appears blue because this blue light is scattered more readily towards us, known as Rayleigh scattering. During sunset the sunlight is still scattered, however the blue light is scattered away from our eyes leaving the oranges and reds you see.

So how does this relate to blue icebergs?

In an average iceberg there are large amounts of trapped air bubbles. As air scatters light, passing light through an iceberg scatters all the visible wavelengths of light toward you.

An iceberg formed under water away from the air will have little to no air bubbles within it.
Passing light through such an iceberg will have the weaker red wavelength of light be absorbed by the ice, leaving the more high energy blue light to pass through the iceberg... so you see it as blue.

Mixing coloured light together always makes white; known as colour addition.

Reference: Kane, J. W. Sternheim, M. M. (1988). Physics. 3rd ed. John Wiley & Sons, New York.
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