Blue Light Gets Lost

A Very Long Journey

Think about our last lesson. The sun sits low at sunset. Sunlight takes a much longer path. It travels through a lot of air. This long trip changes the colors. Earth's air is full of gas. Scientists call these gas parts molecules 1. They act like tiny bumpers. Sunlight must pass through these bumpers. A long path means many more bumpers. The light hits more gas molecules.

The Bouncing Blue Light

Sunlight looks white. But it holds all the rainbow colors. Blue light travels in short waves. Short waves hit the gas bumpers easily. They bounce off the gas molecules. They scatter in all directions. Scientists call this Rayleigh scattering 2.

This bouncing makes the daytime sky blue. It also makes the sunset look red. The blue light bounces around the sky. This bouncing changes everything.

Short Waves Bump More

Wavelength is the distance from crest to crest. Blue light has a very short wavelength. Short waves are very busy. They wiggle up and down quickly. This quick wiggling causes many crashes. They hit the nitrogen in the air. They hit the oxygen in the air. They hit the gas. Then they bounce. A short path causes a few bounces. A long path causes millions of bounces. The blue light cannot survive the long trip. It bounces right out of the sky.

Getting Lost in the Bumps

Imagine playing a fun game. You roll tiny blue marbles. You roll them through a thick forest. The trees are the gas molecules. The blue marbles are the blue light. The blue marbles hit the trees. They bounce off to the sides. They scatter everywhere.

The forest is thin at noon. The path is very short. Some blue marbles reach you. You look up. You see blue.

The forest is thick at sunset. The path is super long. The blue marbles hit too many trees. They bounce too many times. They scatter away into space. The marbles lose their way. They never reach your eyes.

The Pinball Machine

Think of a giant pinball machine. The blue light is the silver ball. The gas molecules are the round bumpers. You pull the spring. You shoot the ball.

The pinball machine is short at noon. The ball hits two bumpers. Then it reaches the bottom. You win the game.

The pinball machine is huge at sunset. The ball hits hundreds of bumpers. It bounces left and right. It stays at the top. It never reaches the bottom. The blue light loses the game. It scatters away into the atmosphere.

Here are the steps of the journey:

1. The sun sends out white light.
2. The light enters Earth's air.
3. The light travels a very long way.
4. The short blue waves hit gas molecules.
5. The blue light bounces away.
6. The blue light never reaches your eyes.

Where Does the Blue Go?

The blue light does not vanish. It just scatters away from you. It bounces to other places. People far away might see it. They might see a blue sky. You do not see it. Your sky loses its blue color.

Here is a look at the two paths:

Time of Day	Path Length	Blue Light Action	What You See
Noon	Very short	Bounces a little	Blue sky
Sunset	Very long	Bounces away totally	Blue is missing

A Cool Science Trick

Teachers can show this in class. They use a clear tank of water. They add a little milk. The milky water acts like the air 4. They shine a white light through it. The light travels through the tank.

You can watch the blue light scatter. It bounces out the sides. The water looks pale blue 5. Now look at the end. The light coming out has no blue. The blue light loses its way.

The Missing Piece

This is the big secret of sunsets. The blue light grows tired. It bounces too much. It scatters away. It never reaches you. The long path through the air stops it. The gas molecules block the short waves. The sunset loses its blue light.

Key Terms

• Rayleigh scattering 2 — The bouncing of short light waves off gas molecules in the air.
• Molecule 1 — A tiny part of a gas or liquid that acts like a bumper for light.
• Wavelength — The distance from the top of one wave to the top of the next wave.