Blue Sky – Red Sunset

Whenever it’s not completely filled with clouds, we can see that the sky is blue. As our world turns the sun rises and sets, and as it sets it looks red. These two observations are related.

You will need:

A torch

a transparent container with flat parallel sides (a 10-litre [2½- gallon] aquarium is ideal)

250 millilitres (1 cup) of milk

Method

Place the container on a table where you can view it from all sides. Fill it ¾ full with water. Shine the torch through the water and you may be able to see some particles of dust floating in the water. However, it is rather difficult to see exactly where the beam passes through the water.

Add about ¼ of the milk to the water and see that the beam of light is now easily visible as it passes through the water. From the side, the beam appears slightly blue, and on the end, it appears somewhat yellow. Add another ¼ of the milk to the water and stir it. Now the beam of light looks even more blue from the side and more yellow, perhaps even orange, from the end. Add the rest of the milk to the water and stir the mixture. The colours should intensify and the beam should spread out.

What’s Happening?

Light usually travels in straight lines, unless it encounters the edges of some material. When the beam of a torch travels through air, we cannot see the beam from the side because the air is uniform, and the light from the torch travels in a straight line. The same is true when the beam travels through water, as in this experiment. However, if there should be some dust in the air or water, then we can catch a glimpse of the beam where the light is scattered by the edges of the dust particles. When you added milk to the water, you added many tiny particles to the water. Milk contains many tiny particles of protein and fat suspended in water. These particles scatter the light and make the beam of the torch visible from the side. Different colours of light are scattered by different amounts. Blue light is scattered much more than orange or red light. Because we see the scattered light from the side of the beam, and blue light is scattered more, the beam appears blue from the side. Because the orange and red light is scattered less, more orange and red light travels in a straight line from the flashlight. When you look directly into the beam of the flashlight, it looks orange or red.

NB the ‘Sunrise’ experiment also available on these pages is a simpler version of this activity.

What does this experiment have to do with blue sky and orange sunsets?

The light you see when you look at the sky is sunlight that is scattered by particles of dust in the atmosphere. If there were no scattering, and all of the light travelled straight from the sun to the earth, the sky would look dark as it does at night.

The sunlight is scattered by the dust particles in the same way as the light from the torch is scattered by particles in milk in this experiment. Looking at the sky is like looking at the torch beam from the side: you’re looking at scattered light that is blue. As the world rotates we see a daily setting sun. It’s like looking directly into the beam from the torch: you’re seeing the light that isn’t scattered, namely orange and red. What causes the sun to appear deep orange or even red at sunset or sunrise? At sunset or sunrise, the sunlight we observe has travelled a longer path through the atmosphere than the sunlight we see at noon. Therefore, there is more scattering, and nearly all of the light direct from the sun is red.

Adapted from and thanks to www.wisc.edu./sci