The Red Sunset

The sun goes down at the end of the day. It sits low on the horizon. The sunlight must travel very far to reach your eyes. It passes through a thick blanket of air. This air is our atmosphere. The air holds many gas molecules. These tiny parts of air bounce the light. Scientists call this bouncing action Rayleigh scattering 1. Lord Rayleigh was a smart scientist. He figured out this bouncing rule a long time ago. Let us look at the light path. The path changes at the end of the day.

Losing the Blue Light

Think about the colors of the rainbow. Sunlight holds all seven colors. We learned about short blue light waves. They bump into the gas molecules easily 2. The air scatters the blue light everywhere. The light bounces like a pinball.

At noon, the sun sits right above you. The light takes a short path. Only some blue light scatters. This makes the sky look blue.

Sunset is very different. The light takes a very long path. It travels through much more air. Almost all the blue light bounces away. It scatters away from your eyes. The blue light gets lost in the sky.

Red Light Pushes Through

Red light waves are long. They do not bump into the air molecules. They push right through the thick air.

Imagine a run through a crowded room. A small dog bumps into many legs. That is like the blue light. A large horse walks straight through. That is like the red light.

The red light does not bounce away. The red light passes through the long air path. The orange light passes through too. The blue light is gone. You only see the red and orange colors 1. This makes the sunset look like fire.

Comparing Noon and Sunset

Let us look at the differences. The time of day changes the sky color. The thickness of the air is the key.

Teachers can show this in a classroom. They use a bright light and a clear tank. They fill the tank with water. They add a little milk to the water. The milk acts like the gas molecules. The light shines through the tank. The light looks blue on the side. The light looks red at the far end 3. This is a great sunset trick.

The Giant Filter

The thick air acts like a giant filter. A filter catches some things. It lets other things pass.

You use a filter to drain wet noodles. The water falls through the holes. The noodles stay in the bowl. The sky works just like that noodle filter.

The thick air catches the blue light. The red light falls right through. This is why the sunset looks so beautiful. The sky glows with warm red colors. Sometimes you see bright orange too. Other things in the air change these colors. Dust and clouds make the sky look different. We will learn about dust and clouds next.

The Speed of Light

Light travels very fast. It zips through space at top speed. But space is very big. The light from the sun takes time. It needs eight minutes to reach Earth 4.

The light flies through empty space. Space has no air. Space has no gas molecules. The light stays perfectly white in space. It holds all the rainbow colors together. Nothing scatters the light in space.

The scattering only starts near Earth. The light hits our air layer. Then the bouncing begins. At sunset, the bouncing happens a lot. The white light loses its blue parts. Only the red parts finish the trip. You see a beautiful red sky.

Key Terms

• Rayleigh scattering 1 — The bouncing of light off tiny gas molecules in the air.
• Filter — A tool that catches some things and lets other things pass through.
• Horizon — The line in the distance where the sky seems to meet the ground.