Colour and filters
Each colour is a band of wavelengths
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)
- Green light, λ = 530 nm, speed 3.00 × 10⁸ m/s
- 530 nm = 5.30 × 10⁻⁷ m
- f = 3.00 × 10⁸ ÷ 5.30 × 10⁻⁷ = 5.66 × 10¹⁴ Hz
Reflect, absorb or transmit
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.
Filters and coloured light
A filter transmits its own colour and absorbs the rest.
- A blue jumper is lit by red light only
- it can only reflect blue light
- there is no blue in red light, so the red light is absorbed
- 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.
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