KS3 · Physics

How surfaces reflect colour

Shine only green light on a bright flag and its colours change. By the end of this lesson you'll be able to predict every one before you see it.

Physics · Light and colour

What does each surface send back?

Here's a simplified model: white light is a mix of three rays, one red, one green and one blue. A surface 'chooses' which of those rays it reflects and absorbs the rest. For each surface, tick every set it reflects. Two clues: a surface that looks yellow sends red and green to your eyes, and one that looks cyan sends green and blue. Tick 'In none' if a surface reflects none of the three.

  • A Reflects red
  • B Reflects green
  • C Reflects blue
  1. Red surface
  2. Yellow surface
  3. White surface
  4. Green surface
  5. Magenta surface
  6. Black surface
  7. Blue surface
  8. Cyan surface

Seeing colour

?

Reason it through

How do you see the colour of a leaf lying in the sunshine?

Link 1 of 4

First link · your turn

Start with the light. Sunlight is called white light. What do you think that means about what is falling on the leaf?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

Physics · Light and colour

Why is the leaf green?

A green leaf is lying in bright sunshine.

Four people give a reason it looks green. Which is closest to what you think, and how sure are you?
How sure are you?

Physics · Light and colour

Same surfaces, new light

Sort the eight surfaces by how they LOOK under this light. Then switch the light and ask the same question again.

How does each surface look when only red light shines on it?

Still to sort

Looks red (0)

Only red light comes back to your eyes.

Looks black (0)

No light comes back.

Where the line is: Black here means the surface reflects none of the colours that are arriving, so nothing at all reaches your eyes from it.

8 of 8 still to sort.

The surfaces haven't changed. The light has. Use the simplified red, green and blue model: ask what arrives, ask what the surface can send back, and keep only the overlap.

Predict, then check: the flag of Guyana

In sunlight, every part of the flag shows its usual colour. The red triangle reflects only red light. The black border reflects none. The yellow chevron reflects red and green and absorbs blue. The white border reflects all colours. The green background reflects only green. Now the Sun goes off and only green light shines on the flag.

In green light only, what do the red triangle, the yellow chevron and the white border look like?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

A surface can't add colour. It can only choose which colours to send back.

What you need to know

  • We see a non-luminous object because it reflects light and some of that light enters our eyes. Luminous objects make their own light.
  • An object's colour depends on which colours of light it reflects. The colours it doesn't reflect are absorbed.
  • In the simplified red, green and blue model, red, green and blue surfaces reflect one colour; yellow, cyan and magenta surfaces reflect two; white reflects all three; black reflects none.
  • To find how a surface looks in coloured light, keep only the colours that arrive AND that the surface can reflect. If none are left, it looks black.

The big picture

A surface can't add colour to light. It reflects some of the colours that arrive and absorbs the rest, and what reaches your eyes is what's left. Use the simplified red, green and blue model to work out how any surface looks in white light or coloured light.

Key points

1Bumpy surfaces scatter light in many directions, so we can see them from any angle. A mirror sends light arriving from one direction off in one new direction.
2White light contains all the colours of the visible spectrum. We see colour when light is absorbed by the R, G and B cone cells and they signal the brain.
3A green object looks green because it reflects green light and absorbs the other colours. It does not look green because it absorbs green.
4A coloured object reflects light. It doesn't give out light of its own colour, because then it would be luminous.
5White reflects all colours equally well, so in coloured light it looks the colour of the light. Black absorbs every colour, so it looks black in any light.
6Black is not a colour of light. It's what we experience when no light enters our eyes.

Worked example

Problem

A cyan book and a magenta book sit under a lamp that gives out only green light. Using the simplified red, green and blue model, what colour does each book look?

⚠ Watch out

Easy trap: assuming a white object looks white in any light. White reflects every colour equally well, but it can only send back the colours that actually arrive. Under a red lamp, a white surface looks red.

🧠

Memory hook

Two questions, every time: what colours arrive, and which of those can this surface send back? Only the overlap reaches your eyes. Surfaces subtract colour, they never add it.

✓

Check yourself

Blue shirts and white shorts under a red-only floodlight: what colour is each, and why? Check: shirts black (they reflect only blue, none arrives); shorts red (they reflect all, but only red arrives).

Flashcards

(14)
Why can we see a non-luminous object?
It reflects light, and some of the light it reflects goes into our eyes.
What is scattering?
Reflection from a bumpy surface: rays that arrive in the same direction are reflected in many directions, so you can see the object from any angle.
How does a mirror differ from a dull surface?
Light arriving at a mirror from one direction is all reflected in one new direction, which is why we see images. A dull surface scatters it instead.
What does white light contain?
Light of all the colours of the visible spectrum, the range of colours we can see.
What are the R, G and B cone cells?
Three kinds of cell on the retina at the back of the eye. Each absorbs and detects a range of colours of light and sends a signal along a nerve to the brain.
How does your brain decide which colour you see?
From the levels of signal from the R, G and B cone cells. For example, signals from both R and G give the experience of yellow.
What happens to the colours of light that a surface doesn't reflect?
They are absorbed.
Why does a green object look green?
It reflects green light and absorbs the other colours. It does not look green because it absorbs green.
How is white light shown in the simplified model?
As a red, a green and a blue ray, standing for the three ranges of colours of light that trigger the three kinds of cone cell.
Which colours do yellow, cyan and magenta surfaces reflect (simplified model)?
Each reflects two. Yellow: red and green, absorbing blue. Cyan: green and blue, absorbing red. Magenta: red and blue, absorbing green. (Red, green and blue surfaces reflect just one.)
Is black a colour of light?
No. There is no black light. Black is what we experience when no light enters our eyes.
What does a black surface look like in coloured light?
Black, in any colour of light. It absorbs every colour, so nothing comes back.
What is the rule for how a surface looks in coloured light?
Keep only the colours that arrive and that the surface can reflect. If none are left, it looks black.
What would you see deep in a cave with your head torch off?
Nothing, however long you wait. No light enters your eyes.

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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