Red Light Passes Through

You know that sunlight holds seven colors. You learned about the rainbow colors before. You also learned about the invisible air. Air holds tiny gas molecules. Nitrogen and oxygen fill the sky above us.

During the day, sunlight pours down. The light hits the gas molecules. Blue light bumps into these tiny gases. Blue light scatters everywhere. That makes the daytime sky look blue.

But what happens to the red light? Red light acts very differently. Red light does not bounce away. Red light travels straight through the air. We must look at light waves.

Big Waves and Tiny Waves

Light travels in moving waves. You can measure a wave from crest to crest. Scientists call this distance the wavelength. Every color has a different wavelength.

Blue light has a very short wavelength. The waves are very tight together. Red light has a very long wavelength. The waves are stretched far out.

Think about a giant and a tiny bug. They both want to cross a rocky field. The rocky field is like the sky. The rocks are the gas molecules.

The bug takes tiny, short steps. The bug bumps into every single small rock. The bug must bounce around the rocks. The bug scatters all over the field.

The giant takes huge, long steps. The giant steps right over the small rocks. The giant walks in a perfectly straight line. The giant crosses the field easily.

Blue light acts just like the tiny bug. Red light acts just like the giant. Red light steps right over the tiny gas molecules.

Light Color	Wave Size	Acts Like	What Happens in Air
Blue	Short	Tiny bug	Bumps and scatters
Red	Long	Giant	Passes straight through

Scientists Measure Light

Scientists study light every day. They use special tools to look at waves. They use bright red lasers. These lasers teach us about light.

In plain terms: scientists measure waves with red lasers. They teach students about wave size and light energy.

Red lasers shoot straight, bright beams. The red light does not scatter easily. The long waves keep the beam very tight. The beam cuts right through the room. You can see the red dot far away.

Red Light Goes Through Things

Red light passes through air very well. It passes through other solid things too. Scientists tested red light on real human bodies.

They shined different color lights on skin. The blue light stopped right at the surface. The red light went much deeper inside.

Doctors found amazing things about red light. Red light can pass deep through skin. Red light can even pass through hard bone 2. The long waves just keep going straight.

This helps doctors treat sick people today. They use special red lights for deep healing. The red light reaches deep places inside the body. Blue light cannot reach those deep places.

How Our Eyes Feel Light

Our eyes notice the difference between colors. Blue light and red light feel different to us. Doctors tested human eyes with light.

They shined blue light into human eyes. They shined red light into human eyes. They carefully measured the feelings in the eyes.

The blue light made eyes feel sore faster 3. The red light felt much more gentle. Long red waves are easy on our eyes.

This shows the amazing power of light waves. Short blue waves scatter and bounce hard. Long red waves travel smoothly and gently. Our eyes like the smooth red waves.

The Long Trip at Sunset

The sun moves across the sky all day. In the late afternoon, the sun sits low. It touches the far horizon.

The sunlight must travel a very long way now. It travels through a lot of thick air. The air holds many tiny gas molecules.

The blue light hits the thick air. It bumps into the gas molecules. It scatters away. It never reaches you.

The red light takes its giant steps. It steps over the thick air. It travels all the way to your eyes.

Soon, you will learn about the red sunset. You will see the sky change color. The giant steps of red light paint the evening sky.

Key Terms

• wavelength 1 — The distance from the top of one light wave to the top of the next wave.
• red light — A color of light with long waves that takes giant steps over gas molecules in the air.
• scatter — To bounce off tiny things and go in many different directions.