Light scattering through food colouring

Aren’t rainbows just the most beautiful things? All the possible colours of light packed in together. Here’s one way you can make your own rainbow at home!

Rainbows at home

I’ve talked about this experiment before. You can read more here. I think it makes some lovely colours and looks great so thought I would revisit it. This time I’ve added in a few colours to try and get the whole rainbow in!

A rainbow you can make at home

How to make your rainbow

This one is really simple.

  • Place your cups,
  • add a few drops of food colouring to every other cup,
  • top every other cup up with water and then dangle a folded piece of absorbent paper like tissue or kitchen roll into each cup.

The tissue should have one end in a full cup and one end in an empty cup.

The liquid will start to soak up the tissue and drip into the empty cups, mixing the colours up. I’ve used about 5 drops of food colouring for the 30 mL of water these plastic shot glasses can hold. Scale up if you make this larger!

Colours soaking through the tissue to make a rainbow

An Easy Experiment is a Good Experiment

One reason I like this experiment so much is that you just set it up and leave it. There’s no reason you can’t all have a go at home. Make it your own! The rainbow is an obvious choice, and is a symbol of acceptance of everyone and being nice and tolerant. You can try any colours in any order you like.

You could try and use paint instead of food colouring. The colour might split as it soaks through the tissue and you might end up with some interesting mixes.

I’ve explained why the water moves before, so read this first.

Unexpected results

If you’re lucky you might also see some colours scatter light – this means they might look different depending on which direction you look at them from. The purple/blue mixture in the gif above is scattering the light and making it look a little like it is glowing but notice that it is more purple at the top and more blue at the bottom? Keep your eyes peeled for that!

What’s going on?

Let’s talk a little bit about the light scattering you can look out for with some of these colours. I get it often with blue.

Light scattering

When the light is shining straight at the cup we have a normal, single blue colour.

Blue colouring in a cup

But when the light shines through from the side or the top, we get a brilliant pink colour too!

The blue colour scatters light and looks pink where the light hits it to make a rainbow in just one cup!

Light Scattering

This effect is even more noticeable when the light is coming from a single point. To make this happen, I turned off all the lights and taped a piece of cardboard over a small lamp – then I poked a tiny hole in the cardboard to get a single point of light. The gif above is the result!

Normally, light moves through a liquid and some of the colours are absorbed more than others. We see what is left over at the end (in this case, blue). What’s happening here is that the light is being scattered – some colours can bounce off molecules in the food colouring and be thrown out at an angle. We can see the result far more easily when there is just a single point of light because the blue colour is so much stronger normally – when the light comes from all around – but when it is mostly dark except for that one point of light the blue is not so overpowering. We can see the scattered pink as a result!

Make your own Rainbow!

What other food colourings, inks or paints cause this kind of scattering? Make the best rainbow you can or investigate strange colours and scattering at home and share them with me with #XPeriMas on Twitter.


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