White Light and the Spectrum
White light is a mixture of all visible colours. When it passes through a prism it refracts (bends) and splits into a spectrum because each colour has a different wavelength.
ROYGBIV: Red – Orange – Yellow – Green – Blue – Indigo – Violet
Colour of Objects
An object's colour depends on which wavelengths it reflects and which it absorbs.
| Object | Absorbs | Reflects | Appears |
|---|---|---|---|
| Red apple | All except red | Red | Red |
| Black cloth | All colours | None | Black |
| White paper | None | All | White |
Colour Filters
A colour filter only lets its own colour of light pass through; it absorbs all others.
A red filter in white light
Only red light passes through → everything looks red.
A red object under blue light
No red light present → all light absorbed → object appears black.
Mixing Light (Additive)
Primary colours of light: Red, Green, Blue
Red + Green + Blue = White
Real world: Rainbow after a rain shower
A rainbow forms when sunlight enters water droplets in the air. Each droplet acts like a tiny prism — white sunlight refracts on entry, reflects off the inside of the droplet, and refracts again on exit. Because each colour has a different wavelength, they refract by different amounts and spread into the familiar arc of ROYGBIV colours.
Real world: Stage lighting and coloured spotlights
Theatre stage lighting uses coloured gels (plastic filters) placed over white spotlights. A red gel only transmits red wavelengths — all other colours are absorbed. Stage designers mix beams from red, green, and blue spotlights (additive colour mixing) to create any colour they need on stage, including white.
Real world: Why do clothes change colour in different lighting?
A blue dress appears blue in sunlight (which contains all colours) because it reflects blue and absorbs the rest. Under a sodium street lamp (which emits only yellow/orange light), there is no blue light for the dress to reflect — so it appears dark or dull grey. The dress hasn't changed, only the light source has.
Practice Problems
Why does a green leaf appear green?
Show answer
What colour does a yellow object appear when viewed under red light?