7.8Chapter 7: Light

Colour — Alhussan Physics - Stage 8 Revision Notes & Topical Questions

5 min read 7 parts Lesson 8 of 12 1 quiz

📚 Alhussan Physics - Stage 8 Hub / Topical Revision

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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

Part 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.

Part 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.

Part 3 of 5Colour filters

Filter

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.

Part 4 of 5Combining filters

1Red light through a green filter

No light gets through.

2Two primary colour filters together

No light passes, so white light looks black.

3Black

The brain perceives no light as black.

4Missing colours

A filter can only remove colours; it cannot produce a colour that is missing from the incoming light.

Part 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.

Key terms

Term	Meaning
Primary colours	Red, green and blue light, which cannot be made from other colours.
Secondary colours	Yellow, cyan and magenta, made by adding two primary colours.
Filter	A material that transmits some colours of light and absorbs others.
Transmit	To let light pass through.
Absorb	To take in light so that it is not transmitted or reflected.