Colour, Filters and Lenses

This lesson covers the behaviour of light with respect to colour, filters, and dispersion — as required by the Edexcel GCSE Physics specification (1PH0), Topic 4: Waves. You need to understand how white light produces a spectrum, how colour filters work, and why objects appear different colours in different lights.

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Dispersion of White Light

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

How Dispersion Works

1. White light enters the prism and is refracted (bends towards the normal because glass is denser than air).
2. Each colour (wavelength) of light refracts by a slightly different amount because each colour travels at a slightly different speed in glass.
3. Violet light is refracted the most (it has the shortest wavelength and slows down the most in glass).
4. Red light is refracted the least (it has the longest wavelength and slows down the least in glass).
5. The light spreads out into a spectrum of colours: red, orange, yellow, green, blue, indigo, violet.

flowchart LR
    A["White light"] --> B["Glass prism"]
    B --> C["Red<br/>(least refracted)"]
    B --> D["Orange"]
    B --> E["Yellow"]
    B --> F["Green"]
    B --> G["Blue"]
    B --> H["Violet<br/>(most refracted)"]

    style A fill:#f0f0f0,color:#000
    style B fill:#95a5a6,color:#fff
    style C fill:#e74c3c,color:#fff
    style D fill:#e67e22,color:#fff
    style E fill:#f1c40f,color:#000
    style F fill:#27ae60,color:#fff
    style G fill:#2980b9,color:#fff
    style H fill:#8e44ad,color:#fff

Exam Tip: Dispersion occurs because different wavelengths of light travel at different speeds in glass. Violet is refracted the most, red the least. The key word is dispersion — use it in your answer.

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Primary and Secondary Colours of Light

Primary Colours

The three primary colours of light are:

• Red
• Green
• Blue

These three colours can be combined in different proportions to produce any other colour of light, including white.

Colour Addition

When primary colours of light are added together (superimposed):

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

The colours produced by mixing two primary colours are called secondary colours: yellow, magenta, and cyan.

Exam Tip: Colour addition applies to light (e.g. overlapping spotlights or pixels on a screen). This is different from mixing paints, which involves colour subtraction. Do not confuse the two.

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Colour Filters

A colour filter is a transparent material that absorbs certain wavelengths (colours) of light and transmits (allows through) others.

How Filters Work

• A red filter transmits red light and absorbs all other colours.
• A green filter transmits green light and absorbs all other colours.
• A blue filter transmits blue light and absorbs all other colours.
• A yellow filter transmits red and green (which make yellow) and absorbs blue.
• A cyan filter transmits green and blue and absorbs red.
• A magenta filter transmits red and blue and absorbs green.

White Light Through a Filter

When white light (which contains all colours) passes through a filter:

Filter Colour	Colours Transmitted	Colours Absorbed
Red	Red	Orange, yellow, green, blue, violet
Green	Green	Red, orange, yellow, blue, violet
Blue	Blue	Red, orange, yellow, green, violet
Yellow	Red + Green	Blue
Cyan	Green + Blue	Red
Magenta	Red + Blue	Green

Coloured Light Through a Filter

If coloured light (not white) passes through a filter:

• If the filter transmits that colour → the light passes through.
• If the filter does not transmit that colour → no light passes through (the object appears black).

Example: Red light through a green filter → the green filter absorbs red → no light passes through → appears black.

Example: Red light through a red filter → the red filter transmits red → red light passes through.

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Colour of Opaque Objects

An opaque object does not transmit light — it either absorbs or reflects it. The colour of an opaque object depends on which wavelengths of light it reflects and which it absorbs.

Rules

• A red object reflects red light and absorbs all other colours.
• A white object reflects all colours equally.
• A black object absorbs all colours and reflects none.

Objects in Coloured Light

The apparent colour of an object depends on the colour of the light illuminating it:

Object Colour	Light Colour	Appears
Red object	White light	Red (reflects red from the white light)
Red object	Red light	Red (reflects the red light)
Red object	Green light	Black (no red light to reflect — absorbs green)
Red object	Blue light	Black (no red light to reflect — absorbs blue)
White object	Red light	Red (reflects all light — only red is available)
White object	Green light	Green
White object	Blue light	Blue
Green object	White light	Green
Green object	Red light	Black
Blue object	Yellow light	Black (yellow = red + green; blue object absorbs both)

Exam Tip: This is one of the most commonly tested unfamiliar scenarios in the exam. The rule is simple: an object can only reflect the colour it normally appears. If that colour is not present in the illuminating light, the object appears black. Work through each scenario logically.

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Opaque, Translucent, and Transparent

Term	Definition	Example
Transparent	Allows light to pass through clearly — you can see objects clearly through it	Clear glass, water, air
Translucent	Allows some light to pass through but scatters it — objects appear blurred	Frosted glass, tracing paper, thin fabric
Opaque	Does not allow light to pass through at all	Wood, metal, brick, thick card

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Colour Subtraction (Paints and Pigments)

When mixing paints or pigments, the process is colour subtraction (the opposite of colour addition with light):

• Each pigment absorbs (subtracts) certain colours and reflects others.
• When pigments are mixed, each one absorbs its range of colours, and only the colours reflected by both pigments are seen.