Blue sky with clouds, and a starburst sun

Why is the sky blue?

Follow these instructions to find out why the sky is blue and sunsets are red.

  • 26 May 2006 | Updated 2 November 2012

On a clear day, the sky looks bright blue. At dusk, a sunset can show reds, pinks and oranges. Why is the sky blue? What makes the sunset red? Try this activity to find out.

You will need

  • a large glass jar or vase
  • water
  • milk or powdered milk
  • torch
  • dark room.

What to do

  1. Fill the jar with water.
  2. Add a few drops of milk or half a teaspoon of powdered milk.
  3. Switch on the torch and shine down into the water - it should appear blue.
  4. Shine the torch through one side of the jar. The torch should be near the middle.
  5. Go to the other side and look at the light through the water.
  6. The water should appear pink, while the area directly in line with the torch should appear a yellow-orange colour.

What's happening

When you look up in the sky it is actually scattered blue light that you are seeing.

The Sun produces white light, which is made up of light of all colours: red, orange, yellow, green, blue, indigo and violet - these are the colours you find in a rainbow. Light is a wave and each of these colours corresponds to a different frequency, and therefore wavelength, of light.

The seven colours of the spectrum all have different wavelengths and they are arranged accordingly. Violet, indigo and blue light have a higher frequency - which means shorter wavelength - than red, orange, and yellow light.

When the white light from the Sun shines through the Earth's atmosphere, it collides with gas molecules. It is these molecules that scatter the light. Blue light has a high frequency, and it is scattered ten times more than red light.

But why does the sky become pink and red at sunset? In this activity you changed the colour you saw by changing the position of the beam of light. Similarly, the sky appears a different colour depending on the position of the Sun. The water containing milk molecules causes the light to scatter in the same way the molecules in the atmosphere do.

The atmosphere is the mixture of gas molecules and other materials surrounding the Earth. It is these molecules and particles that cause the scattering of light. The most brilliant sunsets occur when there are lots dust and smoke particles in the air - so beautiful sunsets often occur over polluted cities.

This activity was featured in Science by Email.