Light Hits the Air

The Giant Bumper Car Ride

Sunlight travels to Earth every day. It moves fast through empty space. The light travels in straight lines. It carries all the rainbow colors. It brings light and heat to us.

Then the light reaches our planet. Earth wears a thick blanket of air. We call this blanket the atmosphere 1. The atmosphere protects our world.

This air is full of tiny bits. We call these tiny bits molecules. You learned about them before. The air is packed with them.

Light waves zoom into the air. They crash into the tiny bits. It is like a giant bumper car ride. The light cannot go straight anymore.

Light hits a molecule. Then it bounces. It shoots off in a new direction. Scientists call this bounce scattering. The light scatters across the sky.

Scattering means bouncing in all directions. The light hits a bump. Then it scatters away. It is a simple but powerful idea.

In plain terms: A smart man named Lord Rayleigh figured this out. He lived a long time ago. He explained how light bounces off air. Scientists named this bounce after him. They call it Rayleigh scattering 3. It is a very important rule.

Different Waves Bounce Differently

Remember the rainbow colors? We learned the ROYGBIV order before. Each color has a different wave size. You measure waves from crest to crest.

Red light has long waves. Blue light has short waves. This size difference is very important. It changes how they bump into things.

Think about walking on a rocky path. A tall giant takes big steps. The giant steps right over the rocks. The giant does not trip.

Red light is like the tall giant. Red waves are very long. They skip over most air molecules. They ignore the tiny bumps.

Red light does not bump into much. It travels straight through the air. It keeps going in a straight line.

Now think about a tiny bug. The bug crawls on the same path. The bug must climb over every rock. The bug hits every bump.

Blue light is like the tiny bug. Blue waves are very short. They crash into almost everything. They cannot step over the molecules.

Blue light hits the tiny air bits. Then the blue light scatters. It bounces all over the place. It shoots left and right.

Shorter waves scatter much more easily 2. They cannot skip over the bumps. They hit the air and scatter. The blue light spreads out everywhere.

A Simple Phone Test

You can see this bounce happen. You can do a fun test. You just need a phone and water. You also need a dark room.

Get a clear cup of water. Add one drop of milk. Stir the water gently. The milk acts like tiny air molecules.

Turn on your phone flashlight. Shine it through the cup. Look at the water from the side. Do not look straight at the light.

You will see a pale blue glow. The blue light bounces off the milk. The red light goes straight through. The milk scatters the blue light.

Scientists use phones for tests like this 4. The phone light acts like the sun. The milky water acts like the sky. It is a great model.

This test shows scattering in action. The light hits the bits. The light bounces away. You can see the science work. It is very easy to do.

The Big Bounce

Light bounces off air molecules every day. This bounce changes what we see. It paints the sky above us.

We will learn about the blue sky soon. We will see why sunsets look red. The bouncing light causes all these colors.

For now, just remember the big bounce. Light waves hit the air. The tiny bits get in the way. The light must react.

Short waves bump and bounce. Long waves pass right through. This is the secret of the sky.

Light Color	Wave Size	Does It Bump?
Red Light	Long	Skips over bits
Green Light	Medium	Bumps sometimes
Blue Light	Short	Bumps and bounces

Here is a picture of the bounce:

The air is full of these molecules. The light hits them constantly. The sky is a giant pinball machine.

The blue light bounces around the most. We will see what happens next. The scattered blue light makes the sky beautiful. It is a wonderful trick of nature.

Key Terms

• Scattering 3 — When light hits a tiny bit of matter and bounces off in a new direction.
• Atmosphere 1 — The thick blanket of air that surrounds our planet.
• Molecules 3 — The tiny bits of matter that make up the air and everything else.