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Sky Blue, Sunset Red

Here's a simple way to demonstrate how particles in the air affect visibility.

Fill three glass jars with cold water. Add nothing to the first jar, one teaspoon of milk to the second jar, and three teaspoons of milk to the third jar. Darken the room.

Direct a flashlight beam through the side of the first jar. The water will appear transparent with no apparent color. Have someone move the flashlight so the light shines toward you through the first jar. The water will still be transparent with no color. Now do the same with the second and third jars. The apparent color in both will be blue. But when the light is shone directly through the water towards you, it will appear yellow in the second jar and pink in the third jar.

Milk is an excellent representation of the sky, because, like the sky, it has tiny particles suspended in it. The colors produced depend on the number of particles and the angles at which the light hits them. Light is scattered when it hits the particles in the atmosphere and in the milk. When there is more milk mixed with water, more of the blue light is scattered, and the direct beam becomes more reddish.

(Remember that visible light, such as sunlight, looks white but is really made up of a rainbow of colors. Different things absorb or reflect these colors in different ways. The molecules in air scatter much more of the sun's blue light than any other color. That's why a clear sky looks blue.)

[Source: Environmental Resource Guide, Air Quality. Air & Waste Management Association]

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