KS3 · Physics

Seeing in colour

The yellow on your phone screen isn't made by any yellow pixels — it's red and green, fooling your eyes.

Mix it with light

Three spotlights, one white wall

A red, a green and a blue spotlight, all equally bright, overlap on a white wall. For each colour below, tick the spotlights whose light lands on that part of the wall to make it. Make your prediction first, then check.

A Red light
B Green light
C Blue light

Red
Green
Blue
Yellow
Cyan
Magenta
White
Black

Light isn't paint

What does your gut say?

You shine a red torch and a green torch, equally bright, onto the same spot on a white wall.

Which is closest to what you think you'd see where the two beams overlap?

A muddy brown, like mixing red and green paint.Yellow. The two lights add together to make a new colour.Darker, maybe even black. The two colours cancel each other out.Red and green side by side. Lights don't really blend.

How sure are you?

I'm sureI think soI'm guessing

A muddy brown, like mixing red and green paint.

This is a common misconception

That's paint thinking, and it's the most common trap in this topic. Nothing gets stirred into a blob here. Each torch adds its own light to the spot. Red light plus green light, in equal amounts, gives yellow, and it's brighter than either torch on its own.

Yellow. The two lights add together to make a new colour.

This matches the evidence

Spot on. Two primary colours of light in equal amounts make a secondary colour, and red + green makes yellow. Light doesn't follow paint rules. It adds up.

Darker, maybe even black. The two colours cancel each other out.

This is a common misconception

Adding light can never leave you with less light. Black is what you see when no light reaches your eye at all. This spot is getting light from two torches, so it's brighter, not darker, and it's yellow.

Red and green side by side. Lights don't really blend.

This is a common misconception

Both beams land on exactly the same spot, so their light reaches your eye together and you see one colour: yellow. Next you'll see how your eye turns two colours arriving together into a single colour.

Coloured lights add together to make new colours of light, and when all three primaries meet they make white. Paint rules don't apply to light.

How your eye sees colour

Follow one patch of light from the moment it enters your eye.

Rods and cones respondThe retina has two kinds of light-detecting cell. Rod cells detect brightness. Cone cells come in three types: one detects red light, one green light and one blue light.

Zoom into your screen

A screen only has three colours of tiny light to work with: red, green and blue.

Zoom right in on a white cloud in a photo on your screen. Which of the tiny lights in that patch are switched on?

Put it all together

The yellow that isn't there

A photo of a sunflower on your phone looks bright yellow, but the screen has no yellow pixels at all. Explain how you end up seeing yellow. [3 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

In the yellow patch, the red and green pixels are lit and the blue ones are off. (1 mark)
That red and green light reaches the retina and strongly stimulates the red and green cones, while the blue cones are hardly stimulated. (1 mark)
The colour we see depends on how strongly each type of cone is stimulated. Strong red plus strong green with no blue is seen as yellow. (1 mark)

Model answer

In the yellow part of the photo, the red and green pixels are lit and the blue ones are off. Their light reaches my retina together and strongly stimulates my red cones and my green cones, but hardly stimulates my blue cones. The colour I see depends on how strongly each type of cone is stimulated, and that mix (red and green, no blue) is seen as yellow. Red and green light add to make yellow, even though no yellow pixel was used.

Your eye never checks where light came from. It only reads how strongly each type of cone is stimulated. Stimulate the red and green cones and you see yellow.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Three colours of light, three kinds of cell in your eye, and every colour you've ever seen.

What you need to know

The primary colours of light are red, green and blue. All colours of light can be made by mixing them.
Equal amounts of two primaries make a secondary colour: red + green = yellow, green + blue = cyan, red + blue = magenta. All three in equal amounts make white.
Coloured lights add together. They do not mix the way paints mix.
The retina at the back of the eye detects light. Rod cells detect brightness, and three types of cone cell detect red, green or blue light. When no light reaches the retina, we see black.
The colour we see depends on how strongly each type of cone is stimulated. Screens use tiny red, green and blue pixels that change brightness to make different colours.

The big picture

Every colour of light can be made from three primary colours of light: red, green and blue. Equal amounts of two make a secondary colour (yellow, cyan or magenta); all three together make white, and no light at all is black. Light doesn't mix like paint. It adds. Your retina has three types of cone cell, for red, green and blue light, and the colour you see depends on how strongly each is stimulated. Screens use this trick, with tiny red, green and blue pixels that change brightness.

Key points

1Red, green and blue are the primary colours of light.

2Two primaries in equal amounts make a secondary colour: yellow, cyan or magenta.

3All three primaries in equal amounts make white light, and no light is black.

4Light adds up. Paint-box rules don't work for light.

5Three types of cone (red, green, blue) plus rods for brightness. How hard each cone is stimulated decides the colour you see.

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Worked example

Problem

At a school concert, a red, a green and a blue spotlight are all equally bright. The green and blue spotlights overlap on the singer, and the red one is off. (a) What colour does the singer's spot look? (b) The technician switches the blue spotlight off and the red one on, so red and green now overlap. What colour is the spot now? (c) Blackout: every spotlight goes off. What do you see?

⚠ Watch out

Using paint-box rules for light. Red and green light make yellow, not a muddy brown, because light adds up.

🧠

Memory hook

Think of traffic lights: amber sits between red and green, and red + green light make yellow. (It's only a memory trick. Traffic lights don't mix their colours.) Then remember that light ADDS: all three together make white, and none at all is black.

✓

Check yourself

A stage spot is lit by equally bright red and blue spotlights. What colour is it? Which spotlight would you add to make it white?

Flashcards

(13)

What are the three primary colours of light?

Red, green and blue.

What is a secondary colour of light?

A colour made by mixing equal amounts of two primary colours of light.

Red light + green light (equal amounts) = ?

Yellow.

Green light + blue light (equal amounts) = ?

Cyan.

Red light + blue light (equal amounts) = ?

Magenta.

What do you get when you mix red, green and blue light in equal amounts?

White light.

Do coloured lights mix the same way as paints?

No. Coloured lights add together to make new colours of light, including white.

Which part of the eye detects light, and where is it?

The retina, at the back of the eye.

Why do we see black?

No light is reaching the retina.

What do rod cells in the retina detect?

Brightness.

What do the three types of cone cell detect?

One type detects red light, one green light and one blue light.

What decides the colour we see?

How strongly each of the three types of cone is stimulated.

How does a display screen make lots of different colours?

It has tiny red, green and blue lights (pixels) that change brightness to make each colour.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Three spotlights, one white wall

What does your gut say?

The yellow that isn't there

More KS3 Physics topics