Key concepts
Light
Physics
Color
Wavelength

Introduction
Have you ever watched a sunset and wondered why the sky suddenly changes from blue to orange and red? Even during the day the sky can have multiple colors ranging from bright blue to gray or white. Where do these colors come from? In this activity you will find out—and simulate your own sunset in a glass! Ready to watch the sun go down?

Background
The majority of light that reaches Earth from space comes from the sun, which is our planet’s principle light source. Light that comes from the sun is called white light, although it is actually a combination of all colors. You can see these colors in a rainbow when white light is separated into its individual colors by raindrops. Each color is characterized by a certain wavelength as light travels through space in waves. Some wavelengths are shorter and have more energy, such as blue light; some are longer with less energy, such as red light.

If these waves hit an object, they can interact with the object in several ways: they can be reflected back from the object like those from a mirror; they can be absorbed, which means they are captured by the object; or they can be scattered and deflected in different directions.

What does this all have to do with the color of the sky? The answer is that before the sunlight reaches Earth, it first has to travel through our atmosphere, which is a layer of gas made up of tiny molecules of mostly nitrogen and oxygen that surrounds our planet. When sunlight travels through the atmosphere, the white light interacts with these gas molecules and is scattered in all directions. Not all wavelengths of light are scattered the same way, though. Shorter wavelengths, such as blue light, are scattered much more than the longer ones. This scattered blue light is what we see and what makes the sky look blue.

When the sun sets, it is much lower in the sky, which means that the sunlight has to pass through more air in the atmosphere to reach your eyes. This causes greater scattering of the blue light. The remaining wavelengths that you see are now longer ones that get less scattered, such as orange and red. Small particles in the air enhance the scattering effect, which is why on hazy days or when there is air pollution, the sunsets appear even redder. Want to see for yourself? Then try this activity, and turn white light into a sunset!

Materials

• Tall glass or jar (at least 16 ounces)
• Water
• Milk
• Teaspoon
• Bright flashlight that emits white light
• A workplace that can tolerate water spills

Preparation

• Fill your glass or jar to the top with tap water.

Procedure

• Shine your flashlight through the glass of water from the side. When you look at the glass from the front, what does the water look like? What color is it? Can you see the light traveling through the water? Is the light beam narrow or wide?
• Add about one teaspoon of milk to the water and stir until mixed. How does the water look now? Is it still clear or does it look cloudy?
• Take the flashlight and point it at the solution from the side. Does the solution change color? If yes, why do you think the color change happened? What color do you see? Can you still see the light traveling through the solution?
• Instead of shining the flashlight through the side of the glass, now place it on top of the glass and shine it through the solution from the top. What changes when you shine the light from the top through the solution? Does the color of the solution look the same throughout the glass or is it different at the top compared with the bottom? Can you explain your observations?
• Extra: What happens if you add more milk to the water? Repeat the activity with different amounts of milk. Do you still see the same colors as before? What changes when you add more milk to the water?
• Extra: Can you replace the milk with other substances and still get the same results? Try different solutions from your kitchen and find out! What do all these substances have in common?

Observations and results
Did you see a sunset in your glass or jar? With only water in your container, you should have seen a clear solution. Even when you shine your light through, its color and appearance does not change much. You might have seen the light beam traveling through the water, which should have been relatively narrow, as the light does not interact much with the water molecules. If you add milk to the water, the solution changes. Milk is an emulsion, which means it consists of many tiny insoluble fat particles that are dispersed in the solution. This is why, after adding the milk to the water, the solution becomes cloudy.

When you point your flashlight at the milk–water mixture from the side, the light waves interact with the tiny particles in the solution and get scattered the same way sunlight does by gas molecules in the atmosphere. As blue light gets scattered most, your solution should have appeared slightly blue. Shining light from the top through the solution increases the distance the light waves have to travel through the water–milk mixture. That means the blue light can be scattered even more throughout the solution. This is why at the bottom of the glass there is almost no blue light left anymore—it looks yellow or orange instead. With the blue light scattered away, the solution should turn from whitish-blue at the top to a yellow-orange color at the bottom—just like a sunset!

Cleanup
Pour the milk–water solution down the sink, clean up any spills and wash your hands.

More to explore
Light, from ExplainThatStuff!
The Science of Light Spectrum, from Ducksters
Why Is the Sky Blue?, from Scientific American
Why Is the Sky Blue and Not Violet?, from TheCuriousEngineer
Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies