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.
Understanding Make a Rainbow with Water and Light
Light changes its path at a boundary because its speed changes when it enters a new material. In water, light moves more slowly than it does in air. The change is not the same for every part of visible light.
Red light is bent the least, while violet light is bent the most. This difference happens because each color has a different wavelength.
A wavelength is the distance from one wave peak to the next. The glass itself can bend the light too, since light crosses from air into glass, then into water, then back into air.
The shape and position of the glass affect the pattern on the paper. A curved glass acts a little like a lens. It sends rays in different directions, which can make the color bands wider or narrower.
Tilting the glass changes the angle at which light meets the water surface. Small changes can move the projected colors a long way across a wall. A sharp band needs a small, bright source of light.
Direct sunlight works well because the Sun appears as a small bright disk. Cloudy light comes from many directions, so the colors are often faint or blurry.
A sky rainbow needs more than water droplets. The Sun must be behind the observer, while rain or mist is in front. Light enters a round drop, bends, reflects once from the inside surface, then bends again as it leaves.
Only certain rays leave each drop at the right angle to reach an observer. This is why a rainbow appears as an arc.
Different people standing nearby do not see light from exactly the same drops. The rainbow is not a solid object sitting in one fixed place.
When doing the project, pay attention to where the colors appear and how bright they are. Record what happens when the glass is tilted, when the water level changes, or when the paper is moved farther away. Keep one condition the same while changing another, so the result is easier to explain.
Use a white surface because colored paper reflects extra colors that can hide the pattern. Never stare at the Sun or aim concentrated sunlight at eyes. The important idea is that the colored pattern comes from light being sorted by its different bending, not from the water creating new colors.
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.