Light scattering through food colouring

I’ve talked about this experiment before – so you can read more here – but 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!

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. You should end up with one end in a full cup and one end in an empty cup for each piece of tissue.

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 so scale up if you make this larger!

One of the reasons I like this experiment so much is that you just set it up and leave it for a while so there’s no reason you can’t all have a go at home. With the rainbow being a symbol of acceptance of everyone and being nice and tolerant, I think it is an obvious choice but 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.

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?

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

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

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

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

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!

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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