Colour and the Eye
Why does a rainbow arc across the sky after a storm? Why does a red shirt look red, and why do artists mix paints to create new colours? In this lesson you will discover that white light is actually a mixture of many colours, that objects appear coloured because of the light they reflect, and that your eye detects light using specialised cells called rods and cones. You will also explore the surprising difference between mixing coloured light and mixing coloured pigments.
Before You Begin
Think about these questions about colour and light.
Write down your answers before reading on:
- What happens when you shine white light through a prism? What does this tell you about white light?
- Why does a green leaf look green? Is green "painted onto" the leaf, or is something else happening?
- If you mix red, green and blue light together, what colour do you get? What about mixing red, blue and yellow paint?
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
White Light and the Colour Spectrum
Separating light into its parts
Sound Waves
White light from the Sun or a torch is not actually "colourless." It is a mixture of many different colours. When white light passes through a prism, it splits into a band of colours called a spectrum: red, orange, yellow, green, blue, indigo and violet. This happens because each colour bends by a different amount as it passes through the prism — violet bends the most, and red bends the least.
This discovery was famously made by Isaac Newton in the 1600s, but the phenomenon has been observed for thousands of years in rainbows. A rainbow forms when sunlight enters raindrops, refracts (bends), reflects off the inside of the drop, and refracts again as it exits. The water droplets act like tiny prisms, separating white light into its component colours.
The visible spectrum at a glance
| Colour | Approximate wavelength | Bending in a prism |
|---|---|---|
| Red | ~700 nm | Least |
| Orange | ~620 nm | Less |
| Yellow | ~580 nm | Medium |
| Green | ~530 nm | Medium |
| Blue | ~470 nm | More |
| Violet | ~400 nm | Most |
How Objects Appear Coloured
It is all about what is reflected and what is absorbed
Objects do not "have" colour in the sense of carrying coloured paint. Instead, they appear coloured because of the wavelengths of light they reflect and absorb.
When white light hits a red shirt, the dye in the fabric absorbs most of the colours in the spectrum — green, blue, violet and so on — and reflects mainly red light. That reflected red light enters your eye, and your brain interprets the shirt as red.
A white object reflects almost all wavelengths of visible light. A black object absorbs almost all wavelengths, reflecting very little light back. This is why wearing black on a sunny day feels hotter — the fabric is absorbing more light energy and converting it to heat.
What happens if you shine only blue light on a red shirt? The shirt absorbs the blue light (because it is designed to reflect red), and almost no light is reflected. The shirt will appear dark or black under blue light. This is an important principle in stage lighting, photography and art.
How the Eye Responds to Light
Rods, cones and the retina
Your eye is an remarkable detector of light. Light enters through the pupil and is focused by the lens onto the retina at the back of the eye. The retina contains millions of specialised cells called photoreceptors.
There are two main types of photoreceptors:
- Rods detect brightness and work well in dim light. They do not detect colour, which is why everything appears grey in very dark conditions. There are about 120 million rods in each eye.
- Cones detect colour and work best in bright light. There are three types of cones, each sensitive to different wavelength ranges: red, green or blue. Your brain combines signals from these three types of cones to produce the rich range of colours you see. There are about 6 million cones in each eye, concentrated in the centre of the retina.
When light hits a rod or cone, it triggers an electrical signal that travels along the optic nerve to the brain. The brain processes these signals to create the image you perceive. Without light, there is no signal — which is why you cannot see in complete darkness.
Mixing Coloured Light vs Pigments
Additive and subtractive mixing
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).
Common Misconceptions
"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.
"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.
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?
Test Your Understanding
1. What happens when white light passes through a prism?
2. Why does a red shirt appear red in white light?
3. A blue ball is placed under pure red light. What colour will it appear?
4. Which statement correctly describes the difference between mixing coloured light and mixing pigments?
5. A student argues that cones in the human eye work like coloured filters, with each cone only letting through one specific colour. Which statement BEST evaluates this model?
Short Answer Questions
1. Explain why a green leaf appears green in sunlight. Use the terms absorption and reflection in your answer. 4 MARKS
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
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?
Answers
▾MCQ 1
C — When 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
B — A 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
D — A 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
A — Mixing 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
C — The 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.
Boss Battle
Test your knowledge in a rapid-fire quiz battle. Defeat the boss by answering questions correctly!
Mark lesson as complete
Tick when you have finished all activities and checked your answers.