Physics

3.11 - Visible Light & Colour

8 min

The colour spectrum

Why objects look coloured

Colour filters

note 1 of 4

The primary colours of light

Each colour has its own narrow band of wavelengths (and frequencies) within the visible spectrum.
The visible spectrum runs from around 400 nm at the violet end to around 700 nm at the red end.

The three primary colours

The primary colours of light are pure red, green and blue - they cannot be made by mixing other colours.
Adding all three together produces white light.
Other colours are produced by mixing the primaries in different amounts (e.g. red and green light mix to make yellow).

Three overlapping circles of red, green and blue light, with orange, pink and teal where two overlap and white at the centre, labelled primary colours, mixed colour and white light.

note 2 of 4

Why objects appear coloured

When light hits an object, the wavelengths it absorbs, transmits or reflects determine its colour.

Opaque objects

Opaque objects let no light through. They absorb some wavelengths and reflect others.
The colour of an opaque object depends on which wavelengths it reflects most strongly.

Colour of object	Reflects	Absorbs
Black	none	all wavelengths
White	all wavelengths equally	none
A primary colour	that colour	all other wavelengths
A non-primary colour	that colour, and/or the primary colours that mix to make it	all other wavelengths

Transparent and translucent objects

Transparent and translucent objects transmit light - light passes through rather than being absorbed or reflected.
They appear the colour of the wavelengths they transmit most strongly.

Three balls with blue, green and red rays arriving, labelled red object (reflects red wavelengths), white object (reflects every wavelength) and black object (absorbs every wavelength), only the reflected rays bouncing away.

note 3 of 4

Colour filters

A colour filter transmits some wavelengths of light and absorbs the rest, so only certain colours pass through.
A primary colour filter transmits only its own colour. For example, a blue filter transmits only blue light and absorbs the rest.

Three panels in which blue, green and red light arrows reach a red, a green and a blue filter, and only the arrow matching each filter's colour passes through it.

Viewing objects through a filter

A blue object seen through a blue filter looks blue - it reflects blue light, which the filter transmits.
A red object looks black through a blue filter - the red light it reflects is absorbed by the filter, so no light reaches the eye.

Two panels: an eye viewing a blue ball through a blue filter, labelled looks blue, and an eye viewing a red ball through the same filter, labelled looks black and red light absorbed.

Non-primary filters

A filter that is not a primary colour transmits its own colour and the primary colours that mix to make it.
For example, a cyan filter transmits cyan, blue and green light.

Six coloured arrows point down into an oval cyan filter and only three come out below, labelled light, cyan filter and transmits cyan, blue and green light.

note 4 of 4

Cameras use visible light

A camera uses a lens to focus visible light onto a light-sensitive film or sensor, which records the image.

Controlling the light

Lens aperture - Controls how much light enters the camera.
Shutter speed - Controls how long the film or sensor is exposed to light.
Sensor sensitivity - Controls how responsive the sensor or film is to light.

key terms

TermDefinition

Primary colours of lightThe three colours of light (red, green and blue) that cannot be made by mixing other colours.

OpaqueDescribes a material that does not let light pass through it. The light is absorbed or reflected instead.

Colour filterA material that transmits certain wavelengths of light and absorbs the rest.

end of notes

Key points to remember

The primary colours of light

Each colour has its own narrow band of wavelengths (and frequencies) within the visible spectrum.

The visible spectrum runs from around 400 nm at the violet end to around 700 nm at the red end.

The three primary colours

The primary colours of light are pure red, green and blue - they cannot be made by mixing other colours.

Adding all three together produces white light.

Other colours are produced by mixing the primaries in different amounts (e.g. red and green light mix to make yellow).

Why objects appear coloured

When light hits an object, the wavelengths it absorbs, transmits or reflects determine its colour.

Opaque objects

Opaque objects let no light through. They absorb some wavelengths and reflect others.

The colour of an opaque object depends on which wavelengths it reflects most strongly.

Colour of object	Reflects	Absorbs
Black	none	all wavelengths
White	all wavelengths equally	none
A primary colour	that colour	all other wavelengths
A non-primary colour	that colour, and/or the primary colours that mix to make it	all other wavelengths

Transparent and translucent objects

Transparent and translucent objects transmit light - light passes through rather than being absorbed or reflected.

They appear the colour of the wavelengths they transmit most strongly.

Cameras use visible light

A camera uses a lens to focus visible light onto a light-sensitive film or sensor, which records the image.

Controlling the light

Lens aperture - Controls how much light enters the camera.

Shutter speed - Controls how long the film or sensor is exposed to light.

Sensor sensitivity - Controls how responsive the sensor or film is to light.