How to make a rainbow: a Simple DIY Water Prism for St Pattrick’s Day

What you’ll learn:

How to make a rainbow at home using a water prism.

Key takeaways:

Different colors of light travel at different speeds in our water prism, which changes their direction slightly and seperates white light into a rainbow.

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water prism how to make a rainbowLet’s learn how to make a rainbow at home!

When I think of St. Pattrick’s day and Leprechauns, I think of rainbows, and pots of gold. We did a fun science project with Leprechaun gold in another post, so it seems fitting to add rainbows into our St. Pattrick’s day science series.

We see rainbows when there is a special combination of the sky, water droplets, and sunlight. If we are missing one of those two ingredients we either have a regular sunny day or a dreary rainy day. To learn how to make a rainbow at home, we will need to first learn what makes a rainbow.

Why is water important to make a rainbow?

In this project, we will first be making a water prism, since rainbows need water it makes sense to recreate the rainbow at home using nothing but water and sunlight.

When we make a water prism we are bending light and learning about light reflection and refraction. Some colors of light can travel quickly through the water, like red and orange. Other colors of light are slower, like blue and purple. This is because red light can be imagined as a slim and trim runner, while purple is a large beast – both runners are going through an obstacle course of swinging balls. Red can zip easily through, coming out faster and more on track. Purple, on the other hand, has to slow down and really go off course to get through all of those large balls, so it comes out slower and more bent.

Why is sunlight important to make a rainbow?

Sunlight provides, well, the light. Without sunlight, it would be night. You might make the case that you could get a moonbow, but that moonbow is just sunlight reflecting off the moon through water…so you still have sunlight. Of course, you can make a water prism at home and get a rainbow without sunlight, you’ll just need a good strong source of white light instead.

So to make a rainbow we need two ingredients, water, and sunlight. To make a rainbow at home, we need three ingredients, water, a mirror, and sunlight. We will use the mirror to make a water prism and imitate the effects of all those tiny water droplets suspended in the air that we need to make a real-life rainbow.

Project Ingredients:

Flat mirror

Rainbow prism glasses ​(if you want to do this portion)Square glass dish

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How to make a rainbow water prism mirror angleHow to make a rainbow with a water prism.

Making a water prism is easy. Just fill up your glass dish with water, place the mirror in at an angle, and find a sunny spot to reflect the sunlight through the water.

Voila! A water prism!

How to make a rainbow water prism mirror angle

You can make more complicated water prisms by making a trangular tube with plastic, silicon putty, and water – but that is much more involved.

What can I do with my water prism?

Once you made your water prism you can experiment with how the angle of the mirror affects the location and look of the rainbow. How do other liquids affect the rainbow? What happens if you add food coloring into your water prism? Can you make a water prism without the mirror? These are all great extensions to learning about rainbows with your DIY water prism!

Seeing millions of rainbows with prism glasses.

Want to bring a special treat to your Leprechaun science projects? A great add-on to our water prism is prism glasses. You don’t make these but instead can use them to see the world in rainbow color.

prism glasses

Prism glasses are made of diffraction gratings. Diffraction gratings are basically a ton of tiny prisms packed next to each other. Imagine 13,500 prisms lined up in one inch. That is what prism glasses are, tons and tons of prisms right next to each other making rainbows! Prism glasses bring up a great question – do you see only one rainbow through them?

Each prism, or diffraction line, will make a rainbow when white light hits it. This causes you to see rainbows emanating from everything! Again, you can look and see what happens when you look only at colored water. Or you can investigate what happens with halogen lights, LED lights, incandescent lights, etc. I particularly love looking at neon signs when I am using my prism glasses! It is a great way to go from Leprechaun rainbows to solar composition!