Light · Contrast
Why things look coloured
A red jumper under a green lamp is still a red jumper and looks almost black. Colour turns out not to be something an object has, but something it does to the light that lands on it.
Start here
Take a red jumper into a room lit only in green.
A disco lamp fitted with a deep green filter is the only light in the room. A white shirt looks green. A green bag looks green. A red jumper looks almost black, and so does a blue one.
Nothing has been done to the jumper. Why has it stopped being red?
A red jumper is not carrying redness around with it. It has a dye that reflects the red frequencies and absorbs the others, and it can only reflect what arrives. Under a green lamp no red arrives, and the green that does is exactly what the dye absorbs — so almost no light leaves the surface at all.
An object that is not itself a source of light has no colour of its own to give off. What it does is take the light that lands on it and split it three ways: some is reflected back — scattered in all directions, if the surface is rough — some is absorbed, its energy taken up by the material, and some may be transmitted through. What reaches your eye is only the reflected part, and the colour you see is the colour of that.
A red jumper is red in white light because it absorbs most of the other frequencies and reflects mostly the red ones. Notice what that means: the red has to be in the light in the first place. Put the same jumper under a green lamp and there is no red arriving to reflect, and the green that is arriving is exactly what the dye is good at absorbing. Almost nothing leaves the surface, so it looks nearly black.
A white object reflects all frequencies about equally, so it takes on the colour of whatever is lighting it. A black object absorbs nearly all of them, whatever they are, which is why it stays black under any lamp — and why it warms up in sunlight, since the absorbed energy has to go somewhere.
At the bench · one dark room, one lamp, five objects
Change the object. Change the light. Two different questions.
Change a control to begin
A single lamp with a filter in a room with no other light, and one object under it. Set what the object is, and set what colour the lamp is putting out.
Commit first. A red jumper is put under a lamp giving out only green light. What does it look like?
The object
The light falling on it
Falling on the object
—
Reflected back
—
Absorbed
—
its energy warms the object slightly
What you see
—
The figure
Every object under every lamp
| The object | In white light | Under a red lamp | Under a green lamp |
|---|---|---|---|
| White shirt — reflects almost everything | White | Red | Green |
| Red jumper — reflects red, absorbs the rest | Red | Red | Almost black |
| Green bag — reflects green, absorbs the rest | Green | Almost black | Green |
| Blue book — reflects blue, absorbs the rest | Blue | Almost black | Almost black |
| Black card — absorbs almost everything | Black | Black | Black |
Read across a row and you are changing the light. Read down a column and you are changing the object. Every cell is the same one-line rule: what you see is what is both present in the light and not absorbed by the surface. Where those two do not overlap, the answer is black.
Key fact
The colour of an object is what it does to the light landing on it: the frequencies it reflects reach your eye and the rest are absorbed, their energy warming it slightly. What you see is only what is both present in the light and reflected by the surface. A red object under a green lamp looks almost black because there is no red arriving to reflect, and a white object takes the colour of whatever is lighting it.
Think again
“An object has a colour, and the light just lets you see it.”
Colour is a relationship between three things — the light arriving, the surface, and your eye — and not a property sitting in the object waiting to be revealed. What a red jumper actually has is a dye that absorbs most frequencies and reflects the red ones, and that is a fact about what it does rather than about what it is. Change the light and the answer changes with it. This is not a trick of the disco lamp: the same jumper looks slightly different under a supermarket strip light, a candle and midday sunlight, because those three are not putting out the same mixture.
“A red filter turns white light red.”
It removes everything else. A filter is a subtracter: it lets its own colour through and absorbs the rest, which is why the light coming out of one is always dimmer than the light going in, and why stacking a red filter and a green one gives you almost nothing rather than yellow. The energy that does not come out has not been converted into red — it has been absorbed by the filter, which is why stage lighting gels get warm.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Apply the rule
A blue book is put under a lamp giving out only red light, in a room with no other light. What does it look like?
Rung 2 · The one that catches people
A white shirt looks green under a green lamp. Which statement is right?
Rung 3 · Explain
Explain why a red jumper looks red in white light and almost black under a green lamp, using the words reflect and absorb.
Rung 4 · Take it somewhere new
A supermarket lights its meat counter with lamps that put out extra red, and clothes shops are careful to light changing rooms with something close to daylight. Explain the physics behind both choices, and say why a shopper might reasonably object to the first one.
Key note
An object that is not a source of light reflects some of the frequencies landing on it and absorbs the rest, and the energy it absorbs warms it slightly. What you see is only the reflected part, so the colour of an object depends on the light as well as on the surface: it is whatever is both present in the light and not absorbed. A red object under a green lamp looks almost black, a white object takes the colour of the lamp, and a black object absorbs nearly everything and stays black.
Going further
Once colour is understood as absorption, a lot of biology reads differently. A leaf is green because chlorophyll absorbs red and blue strongly and reflects green — in other words, green is the part of sunlight the plant is worst at using, and it is thrown away. Flowers advertise with frequencies their pollinators can see, and many of them carry ultraviolet patterns that guide bees to the nectar and are completely invisible to us. And a polar bear is not white: its hairs are hollow and colourless, and scatter every frequency that lands on them.
Before this lesson
Connects to
At GCSE this becomes
- Absorption, transmission and reflection at a surface for different frequencies, colour filters, and why objects appear black under some illuminations.
Where to next
- Next: Current and circuits
Electric circuits
- Previous: Colour and the spectrum
Ask Mr Badmus AI
Got an object and a coloured lamp, and want to work out what it will look like?
The bench is a teaching model and deliberately simplified. Real light and real dyes involve the whole continuous spectrum rather than three separate colours: a red jumper reflects a band of frequencies around red and some of its neighbours rather than red alone, and a red lamp puts out a band rather than a single frequency, so a real red object under a real green lamp looks very dark rather than perfectly black. Almost black is therefore the honest answer and is used throughout. The colours on screen can only approximate the light being described. The room is treated as having no other light in it, which is what makes the effect this strong; in a room with any white light left, every object shows some of its usual colour.
Lesson content © MrBadmusAI.