Clouds & Particles

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3. Clouds, particles and climate

Worksheet 1: Radiation budget

1. Experiment: Blue sky, orange sunset

You will need:

• An overhead projector
• A glass or beaker with a flat bottom
• Low-fat milk
• Black cardboard large enough to cover the top of the projector with a hole about 5 cm in diameter in the middle.

In this experiment you will make a model of the atmosphere in a glass of water!

Cover the top of the overhead projector with the black cardboard and make sure the hole is in the middle. Fill the glass or beaker with water to approximately 10 cm in height and place the glass over the hole. Turn off the lights in the room and draw the curtains. Turn on the projector.  The light will travel through the water, up to the projector's mirror and from there to the screen on the wall where you should see a small circle of light. 

Unlike in the atmosphere, the light travels through the water without being scattered. To make your glass of water resemble the atmosphere, carefully pour a few drops of low-fat milk into the glass and let it mix into the water.

• The circle of light on the wall becomes orange-red in colour.
• If you look at the glass of water from the side it appears light blue in colour.

What has happened?

The tiny particles of fat in the milk scatter the light from the projector, just like particles in the atmosphere scatter sunlight. The blue fraction of the light is scattered most and this is why you can see blue light coming out from the sides of the glass. The red fraction of the light is scattered the least, most of it continues straight through the glass of water, up to the mirror and to the wall.

So this experiment explains why the sky is blue and the sunset is red!

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About this page:

Authors: Ellen K. Henriksen and Camilla Schreiner - University of Oslo - Norway.
Scientific reviewer: Justine Gourdeau - LaMP Clermont ferrand - France - 2004-01-13.
Last update: 2004-03-27.