Visible Light
Light, Sound and Waves

Colour separation with light filters

Physics Narrative for 11-14 Supporting Physics Teaching

Filtering light

An easier way to separate out light of different colours is to use coloured filters. For example, when white light passes through a pure red filter all of the other colours in the spectrum are absorbed and only red light passes through it. So only a very narrow band of frequencies get through.

The transmitted light is red in colour and less bright, as the other colours are absorbed by the filter. Some light is removed from the incident beam.

A pure red filter only allows red light through and a pure blue filter only allows blue light through, so if these filters are used together no light can pass through at all.

Filters isolate individual colours by removing the other colours, so this process is often referred to as colour separation by subtraction. That's not a good name, as its a constant fraction of the beam of other colours that is removed by the colour-specific absorber (so the process is better modelled by multiplication by a number less than 1, rather than subtraction). In practice colour filters are not pure, that is they will allow through a small range of colours. The better the filter the smaller the range.

Filters that select a small range of frequencies are quite rare. Much more common are filters that rely on how the eye sees colour. These allow a number of frequencies through, but humans cannot tell the difference. Again each filter removes a constant fration of each colour in the beam; there is more detail in the SPT: Radiations and radiating topic.

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