A rainbow can be made in a classroom or at home with sunlight, a clear glass of water, and a white wall or sheet of paper. This project shows that white light is made of many colors mixed together. When sunlight passes through water, the light changes direction and spreads out into red, orange, yellow, green, blue, indigo, and violet.
It matters because the same science explains real rainbows in the sky after rain.
Key Facts
- Materials: clear glass, water, sunlight, white paper or a white wall, and a table near a sunny window.
- Refraction is the bending of light when it passes from one material into another, such as air to water.
- White light contains many colors: red, orange, yellow, green, blue, indigo, and violet.
- Different colors bend by different amounts in water, which separates white light into a rainbow.
- The speed of light changes in water: v = c/n, where n is the index of refraction.
- Water has an index of refraction of about n = 1.33, so light travels slower in water than in air.
Vocabulary
- White light
- White light is light that contains many colors mixed together, such as sunlight.
- Refraction
- Refraction is the bending of light as it moves from one material into another.
- Spectrum
- A spectrum is a band of colors formed when white light is separated into its different wavelengths.
- Wavelength
- Wavelength is the distance between repeating parts of a wave and helps determine the color of light.
- Index of refraction
- The index of refraction is a number that tells how much a material slows and bends light.
Common Mistakes to Avoid
- Using a colored or cloudy glass, because it can block, absorb, or scatter some colors and make the rainbow hard to see.
- Holding the glass away from direct sunlight, because weak or scattered light may not make a bright rainbow on the paper or wall.
- Forgetting to use a white surface, because colored paper or a dark wall changes how the rainbow colors appear.
- Expecting the rainbow to appear without adjusting angles, because the sunlight, glass, and screen must line up so refracted light lands in the right place.
Practice Questions
- 1 A beam of sunlight enters water with n = 1.33. If the speed of light in air is about 300,000,000 m/s, what is the speed of light in water using v = c/n?
- 2 A student places white paper 30 cm from the glass and sees a rainbow band that is 6 cm wide. If the paper is moved to 60 cm and the spread doubles, how wide is the rainbow band?
- 3 Explain why a clear glass of water can separate sunlight into colors, but a flat white sheet of paper by itself usually cannot.