White light

If you shine torch light onto a sheet of paper, the light from the torch appears white. The light given out by torch lights, light globes and the Sun is white light. Some sources of light, for example coloured neon lights or LEDs, do not produce white light, but coloured light.

If you want to know what colour make-up or clothes will appear under normal circumstances, you need to check the colours in white light. Checking the colours under coloured light can cause them to look very different.

The light spectrum

If a beam of white light from a globe or from the Sun is shone through a triangular prism, a rainbow is produced.

This is because the white light that comes from a light globe or from the Sun is a mixture of many different colours. When white light passes through a prism, it is split up into these separate colours.

Light passing through a prism
There are many colours in a rainbow, but traditionally we describe them as red, orange, yellow, green, blue and violet.

Coloured strip

White light is therefore a mixture of red, orange, yellow, green, blue and violet light. It is our eye that sees this mixture as 'white'.

Coloured filters

Colourless glass lets light pass through it unhindered. This glass is transparent.

Cellophane and coloured glass are also transparent because you can see through them. However, the colour of what you see is changed.

If you look through red cellophane, everything on the other side appears to be shades of red. If you look through green glass, everything appears green.

Green cellophane will only allow green light to pass through it. The cellophane absorbs other colours of light.

For example, green light will not pass through red cellophane. The green light is absorbed by the red cellophane. However, green light will pass through green cellophane.

Looking at light through green cellophane

As a general rule each colour filter (eg glass or cellophane) will only allow light of its own colour to pass through.


White surfaces

When light hits any surface (eg a book, a tablecloth or your clothes) it may be reflected off the surface.

A white sheet of paper reflects almost all the light that falls on it. It reflects white light because it can reflect all the colours of the spectrum that comprise white light.

If red light only is shone onto a white page, the red light is reflected. If green light is shone onto the page, the green light is reflected. White surfaces can reflect all colours of light.

Coloured surfaces

Clothes can be made up of many different colours. We see them as coloured because of the way they reflect light.

A red surface reflects only red light. When white light hits a red jumper, only the red light is reflected. All the other colours in the white light are absorbed by the dyes in the jumper.

A blue tablecloth reflects only blue light. It absorbs red, orange, yellow, green and violet coloured light. The blue light may be reflected into our eyes. This is why we see it as blue.

Some colours are complex. For example, cyan (blue-green) paint reflects a mixture of blue and green light and absorbs other colours. Our brain sees the reflected mixture as cyan.

Coloured pictures

A television or computer screen produces all the colours using three different coloured dots on the screen. They are red, green and blue dots.

Different combinations of brightness of these dots make our brain think it sees all the other colours.

For example, the light given out by red dots and green dots is interpreted by our brain as yellow. The red and green light does not make yellow light. It is our brain that perceives the yellow colour.

Colour magazines and colour printers also produce colour photographs using combinations of coloured dots. The colours they use are usually magenta, cyan, yellow and black dots.

© Commonwealth of Australia, 2003