PHYSICS · Stage 8 · Chapter 7: Light
Physics 7.8 Colour — Primary and Secondary Colours of Light and Colour Filters
5 min read 5 parts Lesson 8 of 12
Lesson summary
Physics 7.8 Colour — Primary and Secondary Colours of Light and Colour Filters — Alhussan International Academy — Physics (Stages 7–9) notes: Explain what happens when coloured lights are mixed and how colour filters work.Primary colours — Red, green and blue; every colour of lig…
- Cannot be made
- Red filter
- Dimmer light
- Not adding colour
- Red light through a green filter
- Two primary colour filters together
Section 1 of 5Primary colours of light
- Primary colours
- Red, green and blue; every colour of light can be made from these three.
- Cannot be made
- Red, green or blue cannot be made from any combination of other colours, which is why they are called primary.
- The eye
- The cells in the back of the eye absorb different amounts of red, green and blue light.
- You perceive different colours when different amounts of the three are detected.
- The eye can distinguish thousands or millions of colours.
Section 2 of 5Secondary colours and adding light
| Mixture | Colour produced |
|---|---|
| Red + green | Yellow |
| Green + blue | Cyan |
| Red + blue | Magenta |
| Red + green + blue | White |
- Secondary colours
- Yellow, cyan and magenta, made by adding two primary colours.
- Screens
- Televisions, computers and phones combine primary colours of light to produce all the colours seen.
Section 3 of 5Colour filters
- Filter = a material that absorbs some colours of light and transmits others.
- Red filter
- White light through it gives red light.
- Red is transmitted.
- All the other colours are absorbed.
- Dimmer light
- Light seen through a filter is dimmer than without it, because some light has been absorbed.
- Not adding colour
- A filter does not add colour; it removes the other colours already present in white light.
- Uses
- Coloured lights in theatres are made by putting filters over white lights.
Section 4 of 5Combining filters
- Red light through a green filter
- No light gets through.
- Two primary colour filters together
- No light passes, so white light looks black.
- Black
- The brain perceives no light as black.
- Missing colours
- A filter can only remove colours; it cannot produce a colour that is missing from the incoming light.
Section 5 of 5Secondary colour filters
- Magenta filter
- It transmits red and blue light and absorbs green light.
- Yellow filter
- It transmits red and green and absorbs blue.
- Cyan filter
- It transmits green and blue and absorbs red.
- Two filters
- List the primaries each transmits; only the colour passed by both gets through, such as green for yellow and cyan together.
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