How to make a rainbow

This activity is part of our Science Lab series. Check out the ANU Science Lab for more experiments! In this activity, you will create a DIY rainbow using water, sunlight, and a small mirror to explore the properties of light, refraction, wavelengths, and optical density. Watch the video above to see how it is done.

How to make a rainbow materials & ingredients

For this activity:

• Clear glass jar or deep bowl filled with water
• Small flat mirror
• Direct sunlight (or a bright, direct torch in a dark room)
• Sheet of white paper or a plain white wall (to act as a viewing screen)

Safety Note: Never look directly into the sun or at the direct reflection of the sun in a mirror.

How to make a rainbow: Step by step instructions

Step 1: Set up the jar and mirror

1. Place your glass jar filled with water on a table or windowsill where direct sunlight is shining.
2. Insert the small mirror into the jar at a slanted angle so it rests submerged in the water facing toward the sunlight.

Step 2: Adjust and project the rainbow

1. Hold your white sheet of paper above and in front of the jar to catch the reflected light.
2. Slowly adjust the angle of the mirror and paper until a rainbow spectrum appears on your paper screen.

Image

How a water rainbow works: The science explained

This experiment demonstrates light refraction, dispersion, and optical density:

• Refraction and bending: Sunlight looks white, but it is actually a combination of all visible colours mixed together. As white light passes from air into water, it slows down and bends (refracts).
• Wavelengths and colours: Each colour of light travels as a wave with its own distinct wavelength. Red light has the longest wavelength and bends the least, while blue and purple light have the shortest wavelengths and bend the most.
• Dispersion: Because each wavelength bends at a slightly different angle when entering and exiting the water, the white sunlight splits into its individual spectral colours.
• Refractive index: The degree to which light bends depends on a material's optical density and refractive index. Short wavelengths interact more strongly with the atoms in water, causing purple to sit at the bottom of the spectrum and red at the top.

Science fair projects & taking it further: Experimenting with variables

• Liquid test (oil vs water): Replace the water in the jar with clear vegetable oil or baby oil. Because oil has a higher optical density and refractive index than water, how does the spread and angle of the rainbow change?
• Distance test: Move your white paper screen closer to and further away from the jar. How does distance affect the size, brightness, and clarity of the rainbow?