LESSON 01 · LIGHT
How does white light become a rainbow?
The short answer
Sunlight looks white, but it is a mix of many colors. When light passes into water or glass, it bends. Each color bends by a slightly different amount, so the colors spread apart and a rainbow appears.
White light is a team of colors
Around 1666, Isaac Newton let a thin beam of sunlight pass through a glass prism. A band of colors appeared on the wall: red, orange, yellow, green, blue, and violet. When he sent those colors through a second prism turned the other way, they combined into white light again. White light is not one color. It is many colors traveling together.
Light bends when it changes material
Light travels more slowly in water and glass than in air. When a beam enters at an angle, the change in speed makes it bend. This bending is called refraction. You can see it when a straw in a glass of water seems to break at the surface.
Each color bends a different amount
In glass and water, violet light slows down a little more than red light, so violet bends the most and red bends the least. Spreading light into its colors this way is called dispersion. The ordered band of colors is called a spectrum.
How raindrops make a rainbow
A rainbow needs the Sun behind you and raindrops in front of you. Light enters each drop and bends, reflects off the back of the drop, and bends again as it leaves. Each drop sends mostly one color toward your eyes, depending on its angle. Millions of drops together draw the arc, with red on the outside and violet on the inside.
Try it at home
Make a rainbow on paper
You will need
- A clear glass filled with water
- A sunny window, or a bright flashlight
- A sheet of white paper
What to do
- Place the glass of water on a table near a sunny window, close to the edge.
- Put the white paper where the light that passes through the glass lands.
- Slowly move the paper and tilt the glass until you see a small band of colors.
- Write down the order of the colors you see.
SafetyNever look directly at the Sun, even through water or glass.
What's happening?
The curved glass and the water refract the light twice: once as it goes in and once as it comes out. Because each color bends by a slightly different amount, the colors land in slightly different places on the paper. If you see red at one edge and violet at the other, you have made a spectrum.
Key words
- Refraction
- The bending of light as it passes from one material into another.
- Dispersion
- The spreading of white light into separate colors.
- Spectrum
- The ordered band of colors: red, orange, yellow, green, blue, and violet.
- Reflection
- Light bouncing off a surface.
Check your understanding
Why can we see a rainbow only when the Sun is behind us?
Light has to enter the raindrops, reflect off their backs, and come back toward us. That only happens when the Sun is behind you and the rain is in front of you.
Which color bends the most in a prism?
Violet bends the most, and red bends the least.
What happens if you send a spectrum through a second prism turned the other way?
The colors can combine back into white light, as Newton showed.