In my new book, How to Find a Rainbow, Reena and Rekha discover that you can only see a rainbow if the sun is shining behind you and it’s raining in front of you.

This ‘re-appearing rainbow’ makes use of some of the same science that explains how rainbows form!

You will need:

• A clear plastic (polythene) bag – ‘ziploc’ or clear plastic document wallets work well.

• The rainbow template you can download below

• Coloured felt tips (ideally all seven colours of the rainbow)

• Black permanent marker (or a felt-tip that is not water soluble)

• Large clear bowl of water

• Scissors

What to do:

Step 1: Colour in the rainbow! Can you remember the order of the colours? An easy way to remember the correct order from top to bottom is to use the acronym ROY G BIV, which stands for Red, Orange, Yellow, Green, Blue, Indigo, Violet.

Step 2: Cut out the box with the rainbow and put the rectangle of paper into the bottom of your plastic bag. If your plastic bag is too small for this, just draw your own rainbow onto a piece of white paper that fits your bag.

Step 3: Use the black permanent marker to trace over the black lines separating the colours of the rainbow. Wait a moment or two to make sure the ink is dry.

Step 4: To make your rainbow disappear, slowly lower the plastic bag into the water, while you are looking down at it from the top. You may have to experiment a bit by angling the paper slightly towards you when you do this to see the effect, but once you’ve got it you’ll be able to do it again easily.

Step 5: To make the rainbow re-appear, simply take the bag out of the water again!

THE SCIENCE:

White light from the sun is actually a mixture of colours known as the spectrum – Red, Orange, Yellow, Green, Blue, Indigo and Violet. 

Scientists use the word “dispersion” to describe the splitting up of white light into the different colours of the spectrum.

When a beam of white light passes through a raindrop, it bends, and bounces in a way that makes it split up into its different colours. The coloured light that bounces out of the raindrop is what we see as a rainbow.

The word “reflection” is used to describe when light bounces off a surface. The bending of light as it goes from one substance from another, like from air into water, is called “refraction”.  This bending of light is why things look distorted if you look at them through a glass of water or a magnifying glass. 

Refraction is also why you can’t see the colours of the rainbow when you put your paper rainbow in its plastic bag into a bowl of water. When the rainbow is outside the water, the light reflects off it and into your eyes, so you can see it. When it is under water and you look at it from above, the light reflecting off the rainbow is bent so that instead of coming out of the bag, it bounces back inside. So, instead of seeing the rainbow, you see a reflection of the water. 

This effect is called “total internal reflection” because the light is reflected INSIDE the water. It’s also why, if you’re ever at the bottom of a swimming pool and look upwards at certain angles, instead of seeing what’s outside the pool, you see a reflection of the floor!