Clouds & Particles

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3. Radiation budget

Worksheet 1

1. Experiment: Blue sky, orange sunset

You need:

• An overhead projector
• A glass or beaker with a flat bottom
• Low-fat milk
• Black cardboard to cover the projector plate, with a hole (ca 5 cm diameter) in the middle

You are going to make a model of the atmosphere in a glass of water!

Cover the plate 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 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 now travels through the water and up to the projector's mirror, and from there to the screen on the wall where you see a small circle of light. So far, 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 blend into the water.

Now you will observe two things:

• The light circle on the wall becomes orange-red
• If you look at the glass of water from the side, you see a light blue color

What has happened?

The tiny particles of fat from the milk scatter the light, just like particles in the atmosphere scatter the sunlight. The blue fraction of the light is scattered most – that is why you can see blue light streaming out from the sides of the glass. The red fraction of the light is scattered least – most of it continues straight through the glass of water, up to the mirror and to the wall. The experiment is a model explaining why the sky is blue and the sunset is red.

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.