Explain how a colour filter works and describe what happens when white light passes through a red filter and then through a blue filter placed immediately after it.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
White light is made up of all the colours of the visible spectrum. Colour filters are used in theatres, photography and scientific equipment to select specific colours of light.
Model answer (5 marks)
A colour filter is a material that absorbs all wavelengths of light except those of a particular colour, allowing only that colour to pass through. A red filter absorbs all colours except red, so when white light passes through it only red light is transmitted.
The red light that emerges then reaches a blue filter. A blue filter absorbs all wavelengths except blue, so it absorbs the red light that arrives.
Consequently no light emerges from the blue filter; the combination appears black or complete darkness.
The red light that emerges then reaches a blue filter. A blue filter absorbs all wavelengths except blue, so it absorbs the red light that arrives.
Consequently no light emerges from the blue filter; the combination appears black or complete darkness.
Examiner tips
- Use the word ‘transmits’ for the colour that passes through each filter.
- Show the sequence: white → red filter → only red → blue filter → absorbs red → no light.
- Mention that the result is black or complete darkness.
Common mistakes
- Saying the filters transmit all colours except one, rather than only transmitting the specified colour.
- Confusing absorption with transmission for the second filter.
- Failing to state that no light emerges after the blue filter.
Mark scheme (5 marks)
- A colour filter only allows light of a single colour (single frequency) to pass through
- A red filter absorbs all colours except red / only transmits red light
- Only red light reaches the blue filter
- The blue filter absorbs red light (because it only transmits blue light)
- No light emerges / the combination appears black / complete darkness results
Key terms in this question
colour filter · white light · colour
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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