Colour and Light

This lesson covers colour and light as required by the Edexcel GCSE Physics specification (1PH0), Topic 5: Light and the Electromagnetic Spectrum. You need to understand the composition of white light, dispersion, primary and secondary colours, colour addition, why objects appear coloured, and how filters work.

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White Light and the Visible Spectrum

White light is not a single colour — it is a mixture of all the colours of the visible spectrum. When white light is passed through a triangular glass prism, it is split into its component colours. This process is called dispersion.

Dispersion

Dispersion occurs because different colours (wavelengths) of light are refracted by different amounts when they pass through a prism:

• Red light has the longest wavelength and is refracted the least.
• Violet light has the shortest wavelength and is refracted the most.

The colours of the visible spectrum, in order of decreasing wavelength, are:

Colour	Approximate Wavelength
Red	~700 nm
Orange	~620 nm
Yellow	~580 nm
Green	~530 nm
Blue	~470 nm
Indigo	~440 nm
Violet	~410 nm

Exam Tip: Remember the order using the mnemonic "Richard Of York Gave Battle In Vain" — Red, Orange, Yellow, Green, Blue, Indigo, Violet. Red has the longest wavelength; violet has the shortest.

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How a Prism Produces a Spectrum

1. White light enters the prism and hits the first surface at an angle.
2. Each colour is refracted by a slightly different amount because each has a different wavelength (and therefore travels at a slightly different speed in glass).
3. The colours spread out inside the prism.
4. At the second surface, the colours are refracted again, further separating them.
5. The result is a continuous spectrum of colours projected onto a screen.

flowchart LR
    A["White light<br/>(all colours mixed)"] --> B["Enters<br/>glass prism"]
    B --> C["Different colours refract<br/>by different amounts"]
    C --> D["Colours separate<br/>inside the prism"]
    D --> E["Colours exit prism<br/>further separated"]
    E --> F["Red (least refracted)<br/>Orange<br/>Yellow<br/>Green<br/>Blue<br/>Indigo<br/>Violet (most refracted)"]

    style A fill:#f1c40f,color:#000
    style F fill:#8e44ad,color:#fff

If you place a second prism (inverted) after the first, the colours recombine to form white light again. This demonstrates that white light is a mixture of all the colours.

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Primary Colours of Light

The primary colours of light are:

• Red
• Green
• Blue

These three colours cannot be made by mixing other colours of light. However, by combining primary colours in different combinations, you can produce any other colour.

Exam Tip: Do not confuse the primary colours of light (red, green, blue) with the primary colours of paint/pigment (red, yellow, blue). They are different. The exam is about light, not paint.

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Secondary Colours of Light

When two primary colours of light are mixed (added together), they produce a secondary colour:

Primary Colours Mixed	Secondary Colour Produced
Red + Green	Yellow
Red + Blue	Magenta
Green + Blue	Cyan

When all three primary colours are mixed together in equal intensities:

$\text{Red} + \text{Green} + \text{Blue} = \text{White light}$Red+Green+Blue=White light

Colour Addition Summary Table

Combination	Result
Red + Green	Yellow
Red + Blue	Magenta
Green + Blue	Cyan
Red + Green + Blue	White

Exam Tip: Learn the three secondary colours: yellow (R+G), magenta (R+B), and cyan (G+B). These are frequently tested. A good mnemonic: "Yellow is Red + Green" — the two warm colours make another warm colour.

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Why Objects Appear Coloured

An object appears a certain colour because of selective absorption and reflection of light.

The Process

1. White light (containing all colours) hits the object.
2. The object's surface absorbs some colours and reflects others.
3. The reflected colours enter your eyes, and your brain interprets this as the colour of the object.

Examples

Object	Colours Absorbed	Colours Reflected	Appears
Red object	All colours except red	Red	Red
Green object	All colours except green	Green	Green
White object	No colours (reflects all)	All colours	White
Black object	All colours (reflects none)	None	Black
Yellow object	Blue, indigo, violet	Red and green	Yellow

Exam Tip: A white object reflects all colours; a black object absorbs all colours. This is why black objects heat up faster in sunlight — they absorb more light energy.

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Colour Filters

A colour filter is a transparent material that transmits (lets through) only certain colours and absorbs all others.

Examples

Filter Colour	Transmits	Absorbs
Red filter	Red light	Green, blue (and other colours)
Green filter	Green light	Red, blue (and other colours)
Blue filter	Blue light	Red, green (and other colours)
Cyan filter	Green and blue	Red
Magenta filter	Red and blue	Green
Yellow filter	Red and green	Blue

What Happens When Coloured Light Passes Through a Filter?

• If the filter colour matches the light colour → light is transmitted (passes through).
• If the filter colour does not match → light is absorbed and the object appears black (no light gets through).

Worked Example

White light shines on a red filter. What colour light comes through?

White light contains all colours. The red filter transmits red and absorbs all other colours. So red light comes through.

Red light shines on a green filter. What colour is seen?

Red light hits the green filter. The green filter only transmits green light and absorbs red. Since there is no green light to transmit, no light comes through. The filter appears black.

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Predicting Colour Appearance Under Different Lights

You may be asked to predict what colour an object appears under a coloured light (not white light).

Rule

An object can only reflect colours that are present in the light shining on it.

Examples