How to make a rainbow

Materials

For this activity, you will need:

• A clear glass jar or deep bowl filled with water
• A small flat mirror
• Direct sunlight (or a bright torch in a dark room)
• A sheet of white paper or a plain white wall

Safety note: Never look directly at the Sun or at its reflection in a mirror.

How to make a rainbow

Step 1: Set up the jar and mirror

1. Place the jar or bowl of water in a location where direct sunlight is shining.
2. Position the mirror in the water at a slight angle so that it reflects the sunlight.

Step 2: Project the rainbow

1. Hold a sheet of white paper above and in front of the jar.
2. Slowly adjust the position of the mirror and the paper.
3. Continue adjusting until a rainbow appears on the paper or nearby wall.
4. Observe the order of the colours in the spectrum.

Step 3: Investigate the rainbow

1. Move the paper closer to the jar and then further away.
2. Observe how the size and brightness of the rainbow changes.
3. Record your observations.

How does it work?

Sunlight appears white, but it is actually made up of many different colours combined together.

When light passes from air into water, it slows down and changes direction. This process is called refraction.

Each colour of light has a different wavelength:

• Red light has a longer wavelength.
• Blue and violet light have shorter wavelengths.

Because each colour bends by a slightly different amount when it passes through water, the colours separate from one another. This process is called dispersion.

The mirror reflects the separated colours out of the water and onto the paper, allowing you to see a rainbow.

This is the same process that produces natural rainbows after rain, when sunlight passes through tiny water droplets suspended in the air.

Science fair project: Take it further

Once you have created a rainbow, investigate how different variables affect the spectrum.

• Replace the water with a different transparent liquid, such as vegetable oil or baby oil. Does the rainbow change in size or colour separation?
• Compare sunlight with a torch. Which light source produces the brightest rainbow?
• Move the paper closer to and further away from the jar. How does the distance affect the size and clarity of the rainbow?
• Test different mirror angles. Which angle produces the clearest spectrum?
• Investigate whether different containers produce different results. Does a bowl create a different rainbow to a tall jar?
• Create a table to record your observations. Which setup produced the brightest rainbow? Which produced the widest spread of colours?