DISPERSION OF LIGHT
White light can be split up into a spectrum of these colours using a triangular block of glass
or acrylic, known as a prism.
Light is refracted when it enters the prism, and each colour is refracted by a different
amount. This means that the light leaving the prism is spread out into its different colours, a
process called dispersion.
DEF: Dispersion is the splitting of light into its component colours when it passes through a
transparent material.
The colours of the white light spectrum are:
Dispersion happens because different colours of light are refracted by different amounts. Red
light is refracted the least, yellow light is refracted more than red, and so on. Violet light is
refracted the most.
COLOURS OF THE VISIBLE SPECTRUM
There are seven colours of the visible light spectrum. In order of their frequency, from the
lowest frequency (fewest waves per second) to the highest frequency (most waves per
second), they are:
PRIMARY AND SECONDARY COLOURS OF LIGHT
There are three primary colours of light: red, green and blue. Light in these colours can be
added (combined) together to make the secondary colours: magenta, cyan and yellow.
The primary colours of light can be added to make secondary colours, or all three can be
combined to produce white light
Each secondary colour of light is produced by mixing two primary colours.
red + blue = magenta
green + blue = cyan
red + green = yellow
All three primary colours add together make white light:
red + green + blue = white
COLOUR SUBTRACTION, ABSORPTION AND REFLECTION
FILTERS
Filters transmit some colours of light and absorb other colours of light. The colours which
are absorbed by the filter are subtracted from the light.
Viewing coloured objects, or images through different coloured pieces of cellophane can
cause objects to appear a different colour. They may even seem to disappear.
This is because the cellophane acts as a filter, partially or completely absorbing some
colours of light, while other colours are transmitted.
Different colours of light transmit or are absorbed by different coloured filters

Colours which are transmitted pass through the filter and come out the other side.

Colours which are absorbed do not pass through the filter.
FILTERS: PRIMARY COLOURS
Red, green and blue filters each transmit a single primary colour. Any other colours present
in the light are absorbed.

A red filter only transmits red light. It absorbs blue light and green light.

A blue filter only transmits blue light. It absorbs red light and green light.

A green filter only transmits green light. It absorbs red light and blue light.
Example 1
If white light shines at a red filter, red light passes through the filter. This is because:

the red light is transmitted

the blue light and green light are absorbed
A red filter only transmits red light. It absorbs blue light and green light.
Example 2
If yellow light shines at a red filter, red light passes through the filter. This is because:

yellow light is a combination of red light and green light

the red light is transmitted

the green light is absorbed
Yellow light is a combination of red and green light. The red filter absorbs the green light,
but allows the red light to pass through.
Example 3
If cyan light shines at a red filter, no light passes through the filter. This is because:

cyan light is a combination of blue light and green light

the blue light and green light are absorbed

there is no red light to be transmitted
Cyan light is a combination of blue and green light. The red filter absorbs blue light and
green light, so no light passes through the filter.
Filters: secondary colours
Cyan, magenta and yellow filters transmit two primary colours. Any other colour present in
the light is absorbed.

A cyan filter transmits blue light and green light. It absorbs red light.

A magenta filter transmits red light and blue light. It absorbs green light.

A yellow filter transmits blue light and green light. It absorbs red light.
Example 1
If white light shines at a cyan filter, cyan light passes through the filter. This is because:

the blue light and green light have been transmitted

the red light has been absorbed
A cyan filter transmits blue light and green light. It absorbs red light.
Example 2
If yellow light shines at a cyan filter, green light passes through the filter. This is because:

yellow light is a combination of red light and green light

the red light is absorbed

the green light is transmitted
Yellow light is a combination of red and green light. The cyan filter absorbs the red light, but
allows the green light to pass through.
Example 3
If green light shines at a magenta filter, no light passes through the filter. This is because:

the green light is absorbed

there is no red or blue light to be transmitted
Green is a primary colour. The magenta filter absorbs the green light and no light passes
through.
SUBTRACTING COLOURS – DIFFUSE REFLECTION
Diffuse reflection from surfaces can also subtract colours from light.
i)
White surfaces reflect all colours of light, which is why they appear white.
ii)
Black surfaces absorb all colours of light, which is why they appear black.
iii)
Coloured surfaces reflect some colours and absorb others.
Black and white surfaces
Black surfaces absorb all colours of light. White surfaces reflect all colours of light.
Primary colours
Red surfaces only reflect red light, blue surfaces only reflect blue light, and green surfaces
only reflect green light.
Secondary colours
The two primary colours that make that secondary colour of light are the ones reflected by a
secondary coloured surface.
Just like filters, it can be useful to think about the primary colours of light when working out
which colour of light will be reflected by a surface.
Example 1
If white light shines at a blue surface, it appears blue. This is because:
i)
ii)
the blue light is reflected and enters the eye
the red light and green light are absorbed
A blue surface only reflects blue light, which is why it appears blue.
Example 2
If cyan light shines at a red surface, no light is reflected and the object appears black. This is
because:

cyan light is a combination of blue light and green light

the blue light and green light are both absorbed

there is no red light to be reflected
Cyan light is a combination of blue and green light. Both these colours are absorbed by the
red surface, so no light is reflected – and the surface appears black.
Example 3
If magenta light shines at a yellow surface, it appears red. This is because:

magenta light is a combination of red light and blue light

the red light is reflected and enters the eye

the blue light is absorbed
Magenta light is a combination of red and blue light. The yellow surface absorbs the blue
light, and reflects the red light – so the object appears red.