Activity 2.7 - Why are sunsets red?

• Understanding that light can be scattered by small particles
• Understanding that scattering is wavelength dependent

• Large transparent plastic or glass container of water,
• Projector or Overhead projector,
• White screen,
• Dropper containing Dettol

The container of water should be placed in front of the projector and a beam of white light passed through it. Alternatively, a circular hole can be cut in a sheet of card and placed on an overhead projector. The container should then be placed over the card to allow the circular beam to pass through the water.

The students should be asked to consider how the on-screen spot of light will change as Dettol is added to the water. The teacher then gradually adds a few drops of Dettol, causing the transmitted light to change colour from yellow, through orange, to red.

The students should be asked to explain what they think is happening and should be invited to examine the colour of light escaping from the sides of the water container (blue light).

• What is the difference, in terms of the wave model, between blue and red light?
• What property of the Dettol determines which wavelength is scattered?
• Why is the sky blue during the day and red at sunrise and sunset?
• Why are clouds white?