Visible light runs from red (wavelength about 700 nm) through orange, yellow, green and blue to violet (about 400 nm). Each colour is a narrow band of wavelengths, so also a narrow band of frequencies. White light is a mixture of all of them.

wave speed = frequency × wavelength
v = f λ (AQA and Edexcel: recall)

1. Green light, λ = 530 nm, speed 3.00 × 10⁸ m/s
2. 530 nm = 5.30 × 10⁻⁷ m
3. f = 3.00 × 10⁸ ÷ 5.30 × 10⁻⁷ = 5.66 × 10¹⁴ Hz

When light hits an object it can be reflected, absorbed or transmitted.

• Opaque objects transmit no light. Their colour is the colour they reflect; the other colours are absorbed. A red apple reflects red and absorbs the rest.
• White objects reflect all colours equally. Black objects absorb all colours.
• Transparent objects (clear glass) transmit light so you see clearly through them. Translucent ones (frosted glass, tracing paper) transmit light but scatter it.

A filter transmits its own colour and absorbs the rest.

1. A blue jumper is lit by red light only
2. it can only reflect blue light
3. there is no blue in red light, so the red light is absorbed
4. nothing reaches your eye from it: it looks black

White light through a red filter and then a green filter: only red reaches the green filter, which absorbs it, so no light comes out.

A filter never adds a colour. It can only take colours away.