Colours and Filters

A world full of colour

Look around you. How many colours can you spot? Red apples, blue skies, green grass, yellow flowers — the world is bursting with colour. But here is a surprising question: where do all these colours actually come from? The answer is hiding inside something you see every day — ordinary white light.

White light is secretly a rainbow

White light, like the light from the Sun, looks plain and colourless. But it has a secret: white light is really all the colours of the rainbow mixed together. The colours are red, orange, yellow, green, blue, indigo, and violet.

When sunlight passes through tiny raindrops, the drops split the white light back into all its separate colours, and we see a rainbow in the sky. A special glass shape called a prism can do the same trick. So a rainbow is not adding colours to the light — it is just unmixing the colours that were inside the white light all along. You can read more about this in our lesson on how rainbows form.

Why things look coloured

If white light contains every colour, why does a banana look yellow and a leaf look green? The answer is about reflecting and absorbing.

When white light shines on an object, the object soaks up (absorbs) some colours and bounces back (reflects) others. The colour that bounces back into your eyes is the colour you see:

• A banana reflects yellow light and absorbs the rest, so it looks yellow.
• A leaf reflects green light and absorbs the rest, so it looks green.
• A red apple reflects red and absorbs the rest, so it looks red.

So objects do not really have colour stored inside them. They simply choose which colours to bounce back to your eyes! If you want to dig deeper, take a look at how we see colour.

What about white and black?

These two are special:

• A white object reflects all the colours back at once. All the colours together look white, so the object looks white.
• A black object absorbs nearly all the colours and reflects hardly any. With almost no light coming back to your eyes, it looks black.

This is also why black clothes feel hotter in the Sun — they soak up so much light energy that they warm up, while white clothes bounce light away and stay cooler.

Coloured filters: gates for light

Now for something really fun: filters. A filter is a piece of see-through coloured plastic or glass, like coloured cellophane or the lens of coloured sunglasses. A filter works like a gate for colours. It lets its own colour through and blocks the other colours.

• A red filter lets red light through and blocks everything else.
• A blue filter lets blue light through and blocks everything else.
• A green filter lets green through and blocks the rest.

So if you shine white light through a red filter, only red light comes out the other side. The filter has stopped all the other colours of the rainbow and let just the red ones pass.

A colour puzzle

Here is where it gets clever. What happens when you look at different coloured objects through a coloured filter?

Imagine looking through a red filter:

• A red apple still looks red. The apple reflects red light, the red filter lets red through, so red reaches your eye. ✓
• A blue ball looks dark, almost black. The ball only reflects blue light, but the red filter blocks blue. With no light getting through, the ball looks dark.
• A green leaf also looks dark. The leaf reflects green, but the red filter blocks green too.

So a red filter only lets you see red things clearly, and makes blue and green things go dark. The filter can only pass a colour if that colour is actually bouncing off the object in the first place. This is a great mystery to test at home!

Putting it together

Let's remember the two big ideas:

1. An object looks a colour because it reflects that colour and absorbs the rest.
2. A filter lets its own colour through and blocks the others.

When you combine them, you can predict exactly what colour something will look through any filter. You are thinking like a scientist!

Try it yourself! 🧪

Experiment 1 — Make your own rainbow. On a sunny day, fill a shallow glass or bowl with water and place it near a sunny window so sunlight shines into the water. Lean a small mirror inside the bowl at an angle so the Sun hits it through the water. Move the mirror gently until a rainbow appears on the wall or ceiling! The water and mirror split the white sunlight into all its hidden colours, just like raindrops do. (Never look directly at the Sun — only at the rainbow on the wall.)

Experiment 2 — Become a colour detective with a filter. Get some pieces of coloured cellophane or see-through coloured plastic — red, blue, and green if you can. Lay out some brightly coloured objects: a red crayon, a blue toy, a green leaf, a yellow pencil. Now look at them through the red cellophane. Which ones still look bright? Which ones go dark? Write down what you predict first, then check. Try the blue and green filters too. You will discover that each filter only lets its own colour shine through — a real piece of physics you can hold in your hand.

So colour is really a story about light: which colours an object bounces back, and which colours a filter lets pass. Next time you put on coloured sunglasses, you will know exactly why the world looks different through them.