How We See Colour

A world full of colour

Look around you. The sky is blue, the grass is green, a banana is yellow, and a fire truck is bright red. The world is bursting with colour. But have you ever stopped to wonder why things have a colour at all, or how your eyes know the difference between blue and red?

The answer is all about light. Without light, there would be no colour at all. Let's find out how it works.

White light is a secret mixture

Sunlight looks plain white. The light from a lamp looks white too. But here is a surprise: white light is not really one colour — it is many colours all mixed together!

Hidden inside white light are all the colours of the rainbow:

• Red
• Orange
• Yellow
• Green
• Blue
• Indigo
• Violet

When all these colours travel together, your eyes mix them up and see plain white. It is a bit like mixing lots of different juices in one cup — you stop seeing each one on its own.

You can actually split white light back into its colours. That is exactly what happens when you see a rainbow in the sky. Tiny raindrops bend the sunlight and spread the colours out so you can see them one by one. To learn more about that, visit how rainbows form.

How your eyes see colour

Now for the clever part. How do you see the colour of an apple or a leaf?

It works in three steps:

1. Light shines on the object — for example, white light from the Sun lands on a red apple.
2. The object absorbs some colours (soaks them up like a sponge) and reflects other colours (bounces them off).
3. The reflected colour travels into your eye, and your brain says "that's red!"

So an object's colour is simply the colour of light it bounces back to you.

A red apple reflects red light and absorbs all the other colours. A leaf reflects green light and absorbs the rest, so it looks green. A lemon reflects yellow light. The object is not really "full" of that colour — it is just the colour it chooses to bounce back.

Why is grass green?

Let's use what we learned. White sunlight, full of every colour, shines on the grass. The grass contains a substance called chlorophyll. Chlorophyll absorbs red and blue light but reflects green light.

That bounced-back green light reaches your eyes, so the grass looks green. The red and blue light is not wasted, though — the plant uses that absorbed energy to make its food. Clever, isn't it?

What about black and white?

• A white shirt reflects all the colours of light back to you. All the colours together make white, so it looks white.
• A black shirt absorbs nearly all the light and reflects almost none. With hardly any light bouncing back, it looks dark.

This is why black clothes feel hotter in the Sun — they soak up more light energy as heat. White clothes stay cooler because they bounce the light away.

No light means no colour

Here is a big idea to remember: colour comes from light.

Go into a room and turn off every light so it is completely dark. Now look at your colourful toys. You cannot see any colour at all — everything is just black! Why? Because there is no light to bounce off the objects and into your eyes.

The toys are still red and blue and yellow. But without light, no colour can reach your eyes. As soon as you switch the light back on, the colours pop right back. Colour and light always work together.

Try it yourself! 🧪

Make your own rainbow. You need a glass of water, a small mirror, a sunny window, and a white wall or piece of paper.

1. Fill the glass with water and stand it on a sunny windowsill.
2. Slide the mirror into the water, leaning it at an angle so sunlight hits it.
3. Slowly tilt the mirror until you catch the sunlight and bounce it onto the white wall.
4. Look closely — you should see a rainbow appear on the wall!

The water and mirror act like a prism. They bend the colours inside the white sunlight by different amounts, spreading them out into red, orange, yellow, green, blue, indigo and violet. You have just split white light into its secret colours, exactly the way raindrops make a rainbow in the sky.

Bonus: Try putting a coloured sweet wrapper over a torch and shine it on white paper. The paper now shows that colour — because the wrapper only lets that one colour of light pass through. Light is a type of energy, and you can explore more about it in light and shadows.