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Where did the colour go?

Here’s a great trick of the light that looks like magic. Put a picture into water, and the colour disappears! And the best thing is, the colour comes back when you take the picture out again.

You will need

• Pen or pencil
• Zip-lock bag
• Paper
• Scissors
• Textas
• Black permanent marker
• Large, white, waterproof container
• Water

What to do

1. Use a pen or pencil to trace the outline of the zip-lock bag onto the paper.
2. Cut about half a centimetre inside the line, and then check that the piece of paper fits inside the bag. If it doesn’t fit, trim the paper until it does.
3. Use textas to draw a picture on the paper and colour it in.
4. Put the paper inside the bag and seal it.
5. Trace the outline of the picture on the bag with permanent marker.
6. Fill the container with water.
7. Hold the bag upright and slowly lower it into the container. You should see the colour draining from the picture!

What’s happening?

Where did the colour go? In this activity, the colour of the picture seems to disappear, but there’s more to it. If you look closely, the inside of the bag looks white, so you can’t see the paper at all! But why does it do that?

Light travels in straight lines, called rays. When light rays pass from water into air, they get bent. You might have noticed how this affects what you see, when looking through a glass of water or looking at a ladder going into a pool. And light also gets bent when it enters water too.

Inside your bag there’s a tiny bit of air. Any light coming from the paper has to go from air to water, so it gets bent. In this case, the light gets bent away, so it can’t reach the top of the ice-cream container, and that means your eyes can’t see this light.

Luckily, there’s no air on the outside of the bag. That means you can see the picture on the outside of the bag clearly. That’s why that outline is still there, not coloured in!

So why does the inside of the bag look white underwater? Light from the bottom of the white container reflects off the bag’s air. As this light bounces off, staying in the water (rather than passing through the air to the paper), it’s known as a ‘total internal reflection’. This reflection looks white because the bottom of the container is white.

This trick depends on the angle you’re looking from. If your head is quite low, you might not see the illusion. So when you’re showing this experiment off to other people, it’s more likely to amaze tall adults than short younger siblings.

Real-life science

Light is used to send internet, telephone and TV signals. It allows this information to travel super-fast, close to the speed of light! Total internal reflection helps to keep light inside the optical cables.

If you’re after more science activities for kids, subscribe to Double Helix magazine!