Primary colours

In Topic 3.3 you learned that there are seven colours in white light. These are the colours that can be seen in the rainbow.

However, there are three colours of light from which all other colours of light can be made. These are called the primary colours. The primary colours cannot be made by mixing any other colours of light.

The primary colours of light are:

The primary colours of light are different from the primary colours in paints. The colours in light mix differently from the colours in paint.

Adding colours of light

The diagram shows what happens when three beams of light, each of a different primary colour, overlap.

You can see the effect of adding the primary colours:

• red + green = yellow
• red + blue = magenta
• blue + green = cyan
• red + green + blue = white

The different colours that you see on a mobile phone, computer or television are all produced from combinations of the three primary colours of light.

When you look very closely at some types of computer monitor, television or phone screen, you can see the individual sources of red, green and blue light.

When you look at a television or phone screen, you see far more colours than just red, green, blue, cyan, magenta, yellow and white. Most screens can display 256 different colours. Changing the brightness of the primary colours makes all these different colours. For example, orange is made by adding red and green, but with the red brighter than the green.

Research has shown that 256 is the maximum number of colours that most people can see.

Subtracting colours of light

You have probably used a filter in your chemistry lessons. Filters are used to remove something from a mixture.

You can also use coloured filters to remove colours from light.

If a transparent piece of coloured glass or plastic is placed in front of a white light, then only light of that colour will be transmitted (get through). All the other colours will be absorbed.

A common example of coloured filters is in traffic lights.

The traffic lights use three identical white lamps. In front of each lamp is a coloured filter for red, yellow or green.

Take the red filter as the example to see how this works.

White light, from the lamp, is made from the seven colours of light: red, orange, yellow, green, blue, indigo and violet.

When these seven colours arrive at the red filter, only red is transmitted. The other six are absorbed. This is shown in the diagram.

This is an example of subtraction of light. White light has had six colours subtracted to leave only red.

In the traffic lights, the yellow and green filters work in exactly the same way. Each of them absorbs six colours and only transmits one colour.

The blue stage light in the picture has a white lamp and a blue filter. If you look carefully, you can see where the white lamp is inside the black case.

The colours of the filters in stage lights can be changed to produce different colours.

Photographers sometimes use coloured filters on a camera to get different effects.

As with numbers, it is possible to subtract colours of light until the end result is zero.

For example, if white light shines on a green filter, only green light will get through. The other colours of the white light are absorbed.

If this green light then shines on a red filter, then no light will get through. That is because green is one of the colours that a red filter absorbs.

Seeing colours

When you look at a non-luminous object, you see the light that is reflected from the object. Non-luminous means the object does not emit its own light.

Look at the flower in the picture.

The flower is seen in white light. The flower looks yellow because it reflects only yellow light. The flower absorbs the other six colours in white light. This is shown in the diagram.

A white object reflects all the colours in white light equally.

A black object absorbs all the colours in white light and does not reflect any.

These three balls appear black, red or white, according to which colours of light they reflect and which they absorb.

Look at the two cubes in the diagrams.

Red objects only reflect red light, and blue objects only reflect blue light. They absorb all other colours.

So, in white light, the red cube appears red and the blue cube appears blue.

In blue light, the red cube appears black: there is no red light for it to reflect, and the blue light is absorbed.

In red light, the blue cube appears black for the same reason – it absorbs the red light and does not reflect any light.