Why are your shadows coloured?

Normally, a shadow looks dark because an object blocks the light travelling towards a surface.

But in this experiment, three different coloured lights are shining towards the same place.

When your hand blocks one light, the other lights can still reach the wall.

That means your shadow doesn't have to be black — it can be a completely different colour!

What happens when coloured light mixes?

Light doesn't mix the same way as paint.

Your screens produce red, green and blue light. Scientists call these the primary colours of light.

When two overlap, they can make new colours:

• Red + green → yellow

• Red + blue → magenta

• Green + blue → cyan

And when red, green and blue light overlap together:

Red + green + blue → white light

This is called additive colour mixing because adding more coloured light makes the result brighter.

So where do the shadow colours come from?

Here's the clever part.

Your hand isn't making a coloured shadow. It is blocking one or more colours of light.

For example, block the red light and the green and blue lights can still reach the wall. Together they appear cyan.

So:

Block red → cyan
Block green → magenta
Block blue → yellow

Block different combinations and you can create even more shadow colours.

Why is this different from mixing paint?

Mix red, green and blue light, and you move towards white.

Mix paints or coloured filters together and they usually become darker because pigments and filters absorb some wavelengths of light.

That's why your Colour Smashers investigation behaves so differently — the paddles remove some light, while your screens are adding light.

Where is this science used?

Look closely at a television, phone or computer screen and you'll find the same idea at work.

Screens use tiny amounts of red, green and blue light in different combinations to produce the huge range of colours you see.

So the strange colours appearing on your wall aren't just a shadow trick — you've built a simple version of the colour system used by screens all around you.

Scientists Say…

Additive colour mixing happens when different colours of light combine.

The important question isn't just:

“What colour is the shadow?”

Ask:

“Which colours of light are still reaching the wall?”

You can see the subtractive system in action in [Colour Smashers], where coloured paddles remove light rather than add it, and watch it happening inside real leaves in [Sneaky Light Absorbers].