Colour and the Eye
In 1860, James Clerk Maxwell produced the first colour photograph, proving that just 3 wavelengths of light can recreate any colour the human eye sees.
● Know
- That white light is composed of different colours
- How objects appear coloured due to absorption and reflection
- That rods and cones in the eye detect light
● Understand
- Why mixing coloured light gives different results to mixing pigments
- How the eye converts light into signals the brain can interpret
- That colour is a property of light, not of objects themselves
● Can do
- Predict the colour an object will appear under different coloured lights
- Explain the difference between additive and subtractive colour mixing
- Use scientific tools to observe and describe colour phenomena
Hold a red apple under a blue LED light and watch it turn almost black: the apple reflects only wavelengths around 700 nm (red), but the LED emits only around 450 nm (blue), there are no red wavelengths to reflect. Colour is not a property of objects but a perception our brains create in response to the wavelengths entering our eyes. Objects appear coloured because they reflect certain wavelengths and absorb others.
Under white light (which contains all visible wavelengths):
- A red object absorbs green and blue light, reflecting red.
- A green object absorbs red and blue light, reflecting green.
- A blue object absorbs red and green light, reflecting blue.
- A white object reflects all visible wavelengths.
- A black object absorbs all visible wavelengths.
The human eye has three types of cone cells, each sensitive to a different range of wavelengths: short (blue), medium (green), and long (red). The brain interprets the relative signals from these three cone types as colour. This is called trichromatic vision.
Under monochromatic light (single wavelength), objects can only reflect that wavelength or absorb it. A red object under blue light appears black because it absorbs the blue light and there is no red light to reflect.
Stage lighting demonstrates colour subtraction dramatically. A white costume under white light appears white. Under red light, it appears red. Under blue light, it appears blue. But if the costume is actually red (dyed with red pigment), it appears red under white light, red under red light, but black under blue light - because the red pigment absorbs blue light. This is why lighting designers must consider costume colours when designing stage lighting. A scene lit entirely in blue will make red costumes disappear into blackness, while blue costumes will pop.
Australian vision science: The University of Melbourne Department of Optometry and Vision Sciences researches how the human visual system processes colour. Australian researchers have contributed to understanding colour blindness, which affects about 8% of males and 0.5% of females. The most common form is red-green colour blindness, caused by defective or missing cone pigments. Australian researchers also study how Indigenous Australian art uses colour relationships and how these are perceived differently by viewers with various forms of colour vision deficiency.
Objects have intrinsic colours that do not change. This is false. The colour of an object depends entirely on the light illuminating it. A red shirt under blue light is not "really red but looks black" - it IS black under that light, because colour is a perceptual property of the light-object-eye system, not an intrinsic property of the object alone. The shirt has a spectral reflectance curve (how much it reflects at each wavelength), but its colour depends on both this curve and the illumination spectrum.
You shine pure blue light on a red apple. Predict what colour the apple appears.
The apple appears black or very dark because red objects absorb blue light and reflect only red light. With no red light to reflect, little light reaches the eye.
Use these terms in your explanation: absorb · reflect · wavelength · colour
There are two fundamentally different ways to mix colours:
Additive colour mixing combines light sources. The more colours you add, the closer you get to white. Primary additive colours: red, green, blue (RGB).
Red + Green = Yellow
Green + Blue = Cyan
Blue + Red = Magenta
Red + Green + Blue = White
Televisions, computer screens, and stage lighting use additive mixing because they emit light.
Subtractive colour mixing combines pigments or filters. Each pigment absorbs (subtracts) certain wavelengths. The more pigments you mix, the closer you get to black. Primary subtractive colours: cyan, magenta, yellow (CMY).
Cyan + Magenta = Blue
Magenta + Yellow = Red
Yellow + Cyan = Green
Cyan + Magenta + Yellow = Black (theoretically; in practice, a separate black ink is added, giving CMYK)
Printing, painting, and textiles use subtractive mixing because pigments absorb light.
When you look at a printed photograph, the colours are created by tiny dots of cyan, magenta, yellow, and black ink. Each dot absorbs certain wavelengths from the white paper. Where cyan and yellow dots overlap, green light is reflected (cyan absorbs red, yellow absorbs blue; only green remains). A high-quality print might use 2,400 dots per inch, too small for the eye to resolve individually, so the brain blends them into continuous tones. In contrast, your phone screen creates colours by emitting different proportions of red, green, and blue light from adjacent pixels. A white screen emits all three at full intensity; a black screen emits none.
Australian colour technology: The Colour and Imaging group at the University of Technology Sydney develops colour management systems for digital imaging, ensuring consistent colour reproduction across devices (cameras, monitors, printers). Australia printing industry uses these standards for everything from magazines to packaging. Australian researchers also study how colour perception varies with age, culture, and lighting conditions, contributing to international colour standards (CIE) that ensure consistent colour communication worldwide.
The primary colours are red, yellow, and blue. This is an outdated and misleading teaching model. For additive mixing (light), the primaries are red, green, and blue. For subtractive mixing (pigments), the primaries are cyan, magenta, and yellow. Red, yellow, and blue (RYB) was a historical pigment model that is inaccurate by modern standards. Magenta is a better subtractive primary than red because it absorbs green light, allowing it to mix with cyan to produce blue and with yellow to produce red. Teaching RYB confuses students and does not match how modern colour systems actually work.
Beyond simple absorption, several other mechanisms produce colour in nature and technology.
Structural colour: Some materials have microscopic surface structures (ridges, layers, or arrays) that interfere with light waves, amplifying some wavelengths and cancelling others. This produces brilliant, often iridescent colours that change with viewing angle. Examples: butterfly wings, peacock feathers, opals, and some beetles. Structural colour does not fade because it does not depend on pigments that can degrade.
Bioluminescence: Some organisms produce light through chemical reactions. Fireflies, glow-worms, and deep-sea fish use bioluminescence for communication, hunting, or camouflage. The light is produced by luciferin reacting with oxygen in the presence of the enzyme luciferase.
Fluorescence and phosphorescence: Some materials absorb light at one wavelength and re-emit it at a longer wavelength. Fluorescent materials glow only while illuminated; phosphorescent materials continue glowing after the light source is removed.
The blue Morpho butterfly wings are not blue because of pigment. They are blue because of nanoscale ridges on the wing scales that reflect blue light through constructive interference while other wavelengths pass through or are absorbed. If you crush the wing, destroying the structure, the blue colour disappears. This structural colour is so effective that the wings appear intensely blue even in dim forest understorey. Engineers study Morpho wings to develop colour displays that do not require pigments or backlighting - just structural surfaces that reflect specific colours.
Australian structural colour research: Australian researchers at Swinburne University have developed artificial structural colour materials inspired by butterfly wings. These materials use nanostructured surfaces to produce vivid colours without dyes or pigments. Applications include anti-counterfeiting features on banknotes (already used on Australian polymer notes), colour-changing security labels, and low-power reflective displays. The Australian Institute for Bioengineering and Nanotechnology at UQ also studies natural structural colours in beetles and birds to understand how evolution optimises optical nanostructures.
All colours in nature come from pigments. This is false. Many of the most vivid colours in nature are structural: peacock feathers, butterfly wings, beetle carapaces, opals, and some bird feathers. These colours arise from physical optics (interference, diffraction, scattering) rather than chemical absorption. Structural colours can be more brilliant than any pigment because they can approach 100% reflectivity at specific wavelengths. They also do not fade because the colour depends on physical structure, not chemical compounds that can degrade. Understanding structural colour expands our appreciation of nature engineering sophistication.
Match each colour phenomenon to its explanation.
There are two very different ways to mix colours, and they give opposite results:
Mixing coloured light (additive mixing): When you combine coloured lights, you are adding more light. Red light + green light + blue light = white light. This is how TVs, computer screens and stage lights work. The primary colours of light are red, green and blue (RGB). Mixing red and green light gives yellow. Mixing all three at full brightness gives white.
Mixing pigments (subtractive mixing): When you mix paints, each pigment absorbs some colours and reflects others. A yellow paint absorbs blue and reflects red and green. A blue paint absorbs red and green and reflects blue. When you mix them, both pigments absorb more light, and less is reflected. Yellow + blue = green (because together they absorb most colours except green). Mixing many pigments together absorbs more and more light, eventually producing black or dark brown. The primary pigments are cyan, magenta and yellow (CMY).
A TV screen produces colours by combining red, green and blue light. What type of colour mixing is this, and what is produced when all three are combined at full brightness?
Wrong: "Colour is a property of objects, like mass or shape." No � colour depends on both the light shining on an object and the wavelengths the object reflects. A red shirt appears black under pure blue light because there is no red light for it to reflect.
Right: Colour is not a fixed property of an object, it depends on the interaction between the object and the light hitting it. An object can only reflect wavelengths that are present in the incident light. Under white light a red shirt looks red; under pure green light it appears black because there are no red wavelengths to reflect.
Wrong: "Mixing red, green and blue paint gives white." No � that works for light, not paint. Mixing red, green and blue paint absorbs more and more light, producing a dark muddy colour. Paint mixing is subtractive, not additive.
Right: Mixing red, green and blue light (additive mixing) produces white because you are combining all visible wavelengths. Mixing red, green and blue paint (subtractive mixing) produces a dark muddy brown because each pigment absorbs certain wavelengths, combining them absorbs nearly everything, leaving little light to reflect.
Wrong: "The eye works like a camera, simply recording what is there." No � the eye detects light and converts it into electrical signals, but the brain constructs what you see. Colour perception also depends on surrounding colours and lighting conditions, which is why the same object can appear different in different settings.
Right: The eye detects light and converts it into electrical signals, but the brain actively interprets and constructs the visual scene. Colour perception involves three types of cone cells sensitive to different wavelengths, plus significant brain processing. This is why optical illusions, coloured backgrounds and different lighting conditions can all alter the colour we perceive.
Colour in Australian Art and Nature
Indigenous art and ochre: Aboriginal and Torres Strait Islander Peoples have used natural pigments for tens of thousands of years. Ochre, a natural clay earth pigment ranging from yellow through red to brown, is mixed with water or animal fat to create paints for rock art, body painting and ceremony. These pigments work by subtractive mixing: each ochre absorbs some wavelengths and reflects others, giving the characteristic earthy colours.
The outback at sunset: The red colours of the Australian outback are caused by iron oxide in the soil, which reflects red and orange light strongly. At sunset, when sunlight travels through more of the atmosphere and blue light is scattered away, the red light reflected by the landscape becomes even more intense.
The great barrier reef: Coral reefs are among the most colourful ecosystems on Earth. Corals and reef fish appear brightly coloured because their surfaces contain pigments that reflect specific wavelengths. However, as divers descend, red light is absorbed by the water first, so red corals appear grey or brown at depth, a direct consequence of how water absorbs different colours at different depths.
✍ Copy Into Your Books
▾White Light and Colour
- White light is a mixture of all colours in the visible spectrum
- A prism or raindrop can separate white light into colours
- Different colours bend by different amounts in a prism
How Objects Appear Coloured
- Objects reflect some wavelengths and absorb others
- A red shirt reflects red light and absorbs other colours
- White reflects all colours; black absorbs all colours
Light Mixing vs Pigment Mixing
- Red + green + blue light = white (additive)
- Cyan + magenta + yellow pigment = black (subtractive)
- Screens use additive mixing; paints use subtractive mixing
Predict the Colour
Light or Pigment?
At the start of this lesson you were shown mixing red and blue paint versus shining red and blue light on the same spot, two completely opposite outcomes showing that colour mixing with light and paint follows entirely different rules.
Now that you've worked through the lesson, how has your thinking shifted? Can you explain that hook idea more precisely using what you've learned today?
Q1. 1. Explain why a green leaf appears green in sunlight. Use the terms absorption and reflection in your answer. 4 MARKS
Q2. 2. A stage performer wearing a white costume is lit with pure red light. Describe what colour the costume will appear and explain why. Then describe what would happen if the performer was wearing a blue costume under the same red light. 4 MARKS
Q3. 3. Aboriginal and Torres Strait Islander Peoples have used natural ochre pigments for art and ceremony for tens of thousands of years. Explain how ochre pigments produce colour using the concepts of absorption and reflection, and compare this with how a digital screen produces the same colour using light. 4 MARKS
Revisit Your Thinking
Go back to your Think First answer. Has your understanding changed?
- Can you now explain what a prism reveals about white light?
- Can you explain the difference between mixing lights and mixing paints?
Model answers (click to reveal)
Answers
▾MCQ 1
CWhen white light passes through a prism, it separates into a spectrum of colours (red through violet). This happens because different colours bend by different amounts as they pass through the glass.
MCQ 2
BA red shirt reflects red light and absorbs most other colours in the spectrum. The reflected red light enters our eyes, and we perceive the shirt as red.
MCQ 3
DA blue ball absorbs red light and reflects blue light. Under pure red light, there is no blue light for the ball to reflect, and the red light is absorbed. With very little light reflected, the ball appears black or very dark.
MCQ 4
AMixing red, green and blue light is additive: more light is added, producing white. Mixing pigments is subtractive: each pigment absorbs some colours, so mixing many pigments absorbs more light and produces black or dark brown.
MCQ 5
CThe model is partially correct but oversimplified. Each cone type is most sensitive to a range of wavelengths (red, green or blue), not just one exact wavelength. The brain combines signals from all three cone types to produce the full range of colour perception.
Short Answer 1
Model answer: A green leaf appears green because its surface contains pigments that absorb most wavelengths of visible light except green. When white sunlight hits the leaf, the pigments absorb red, orange, yellow, blue and violet light, and reflect green light. This reflected green light enters our eyes, and we perceive the leaf as green. If the leaf were placed under pure red light, it would appear dark because there is no green light for it to reflect.
Short Answer 2
Model answer: Under pure red light, a white costume will appear red. White objects reflect all colours, so when only red light is available, the costume reflects that red light and appears red. If the performer wears a blue costume under the same red light, the costume will appear black or very dark. This is because the blue costume absorbs red light (it is designed to reflect blue), and with no blue light available to reflect, almost no light reaches the eye.
Short Answer 3
Model answer: Ochre pigments produce colour through subtractive mixing. The pigment contains compounds that absorb certain wavelengths of light and reflect others. For example, red ochre absorbs green and blue light and reflects red and orange. A digital screen produces colour through additive mixing: it emits red, green and blue light from tiny pixels. By adjusting the brightness of each pixel, the screen can produce any colour. Where ochre subtracts light by absorption, the screen adds light by emission.