Making Rainbows: A simple prism experiment

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Need an easy STEM experiment to get your day started? Look no further!

This experiment involves creating an at-home prism to see a rainbow from a ray of sunshine. It gives you the opportunity to talk to your child about refraction and reflection, how sunshine is made from each color of the rainbow, and practice observation skills!

Ready for an experiment that’s all sunshine and rainbows? Let’s do it!

How to make the Making Rainbows prism experiment

Supplies you will need

For this experiment, you’ll need:

  • Small glass bowl or cup
  • Water
  • Access to sunlight

Here’s a fun book about rainbows and prismsOpens in a new tab. that would be excellent to accompany this experiment!

Supplies needed for the Making Rainbows Inside prism experiment

Before you start

Please do not look directly at the sun.

Instructions

Here is how to do this experiment with your child:

Step 1: Add water to your glass bowl or cup

Choose a glass bowl or cup and add water to it (it doesn’t have to be full).

If you have a choice, opt for the bowl to have a more flat surface area for the sunshine to filter through.

Step 2: Find some sunshine

Head outside and hold your bowl out to catch some sun. The water needs to be fairly still to see a flat rainbow; otherwise, it is distorted and slightly blurry.

Step 3: Hold the container to reveal a rainbow

See if you can find each color of the rainbow! My kids could easily find the more obvious colors, like blue and red/orange, but if you look closely, you can see each color.

The STEM behind the Making Rainbows prism experiment

This experiment teaches:

  • How rainbows come from sunlight
  • Refraction and reflection
  • Observation

How it works

In this experiment, we are creating an at-home prism by filling a bowl or glass with water and shining light through it at a specific angle.

Sunlight is white light, but that light actually contains all of the colors of the rainbow!

Those colors are blended, creating a simple white light for us to see. But when we use water to create a change of direction, called refraction, this is when each color of the rainbow is bent at different angles and we can see them individually!

How rainbows come from sunlight

When we look at the sunlight, it appears to be white, like when you see a sunbeam shining through your window and onto the floor. The sun is actually emitting all of the colors of the rainbow, but our eyes perceive all of those colors blended as white.

Each color of light has a different wavelength. When sunlight enters our makeshift prism, the light slows down a bit due to water and glass being denser than air and is bent (more on that in the next section).

That helps us to see the true color of sunlight, which is a blend of the colors of the rainbow!

Refraction and reflection

When light travels from air into water, it slows down because the water is more dense. This change in speed causes the light to bend, or refract, following a specific angle determined by the water’s refractive index.

Different wavelengths of light (colors) have slightly different speeds in water, which means they also refract at slightly different angles. This causes the single beam of light to separate. It then reflects off the inside, is refracted again, and leaves as separate colors.

Violet light, having the shortest wavelength, bends the most, while red light, with the longest wavelength, bends the least.

Observation

Making a rainbow from a bowl of water requires careful observation from your child. The light must travel through the water at a specific angle to create the rainbow, and once it does, it requires you to look carefully to see each color of the rainbow!

More experiments about rainbows to try out with your child

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