Why is the sky blue?

Why is the sky blue?

Investigate the sky at home!

The students of Lab_13 saw our Why is the sky blue? article and created an experiment to help us explain the theory. Try it out at home!

What you need:

  • A tall smooth glass or clear plastic container (We used a big graduated cylinder but a vase or something similar would work too)
  • A bright torch
  • Semi-skimmed milk
  • Water
  • And (very important) a dark room

How to:

  • Fill the container about three quarters full with water.
  • Add 1 tablespoon of milk and mix well so that the liquid is very slightly clouded.
  • In a dark room, shine the torch from the bottom of the container about 2 inches away. The liquid appears a light blue-grey colour.
  • If we move the torch to the side of the container and look at it from the opposite side, the liquid now appears to be a pinkish orange.
  • Play around with distances, angles and position of the torch to see how the different colours in the sky occur.

What's happening?

We are using the torch, water and milk as a model for what's happening in the sky. The torch is the sun and the milk droplets act as the molecules of gas in the atmosphere. It is these molecules that scatter the sun's light and our torch's light.

It appears blue when we shine the light from the bottom because the blue light is scattered the most as it travels up the container.

We can make this liquid change colour, and investigate red and pink sunsets, by moving the torch and changing the position of the beam of light. In the same way, the sky appears a different colour depending on the position of the sun.

It is often said that the most beautiful sunsets occur over polluted cities and after volcanic eruptions when there is a lot of smoke and ash in the air. We can see this intensity of colour change when we add more milk to the mixture.

For more weather experiments to try at home, go to Weather Labs.