Colour · Grade 4

White light is a mixture of all the colours of the spectrum, from red (longest wavelength, about 700 nm) to violet (shortest, about 400 nm). A prism separates them by dispersion. Objects look coloured because they reflect some colours and absorb the rest; filters let some colours through. Mixing coloured light is different from mixing paint.

Key facts

• Spectrum order: red, orange, yellow, green, blue, indigo, violet. Violet refracts most in a prism.
• An object reflects its own colour(s) and absorbs the others; white reflects all, black absorbs all.
• A filter transmits its own colour(s) and absorbs the rest.
• Primary colours of light: red, green, blue. Red + green = yellow; red + blue = magenta; blue + green = cyan; all three = white.
• In coloured light, an object looks black if none of the colours it reflects are present.
• Frequency of a colour: f = c ÷ λ (c = 3.0 × 10⁸ m/s).

Worked examples

Other years

• Colour · Grade 5
• Colour · Grade 6
• Colour · Grade 7
• Colour · Grade 8