White Light Is Mixture
White light (eg. sunlight) is not a single colour — it is a mixture of all visible colours.
Dispersion: Splitting Light
A prism (or raindrops) bends different colours by different amounts because each colour has a slightly different wavelength. This separates white light into a spectrum.
Order of colours (longest → shortest wavelength):
Red → Orange → Yellow → Green → Blue → Indigo → Violet
A helpful mnemonic: ROY G BIV
How does a rainbow form?
Sunlight enters spherical water droplets, refracts (bends), reflects inside, then refracts again on exit. Each colour bends differently → spread into a spectrum across the sky.
Colour Mixing: Light (Additive)
Mixing coloured lights adds wavelengths.
| Mix | Result |
|---|---|
| Red + Green | Yellow |
| Red + Blue | Magenta |
| Green + Blue | Cyan |
| Red + Green + Blue | White |
Primary colours of light: Red, Green, Blue (RGB)
Colour Mixing: Pigments (Subtractive)
Mixing pigments/paints subtracts wavelengths (each absorbs some colours).
Primary colours of paint: Cyan, Magenta, Yellow (CMY) Cyan + Magenta + Yellow → Black (in theory)
Colour Filters
A colour filter only lets through light of its own colour, and absorbs all others.
Example: A red filter only transmits red light. A white object seen through a red filter looks red. A green object seen through a red filter looks black (no red light to reflect).
Why Objects Have Colour
An object's colour is the colour of light it reflects:
- A red apple absorbs all colours except red — it reflects red.
- A black object absorbs all colours — reflects nothing.
- A white object reflects all colours.
Real world: TV and phone screens (RGB)
Every pixel on an LCD or OLED screen is made from three sub-pixels — red, green, and blue. By mixing different intensities of RGB light, the screen can reproduce millions of colours. Full R+G+B at maximum gives white; all off gives black. This additive mixing is why digital images are described with RGB values.
Real world: stage lighting
A theatre lighting designer uses gels (colour filters) on spotlights. A red gel transmits only red wavelengths — a dancer wearing a green costume appears black under a red spotlight because the costume absorbs red and the green light it would normally reflect is absent. Understanding colour absorption is essential for stage design.
Real world: rainbows
When sunlight enters a rain droplet it refracts, reflects off the inside, then refracts again on exit. Because violet light refracts more than red light (shorter wavelength, higher refractive index in water), the colours separate across approximately 40–42°. The result is a full visible spectrum arc across the sky after a rain shower.
Practice Problems
A blue jumper is placed under red light only. What colour does it appear?
Show answer
What are the three primary colours of light?