Spark image

Prisms and colour

If a beam of light of one colour is shone through a prism, the direction of the beam is changed by the prism. This is because the two faces of the prism through which the light passes are not parallel.

Notice that red light is bent less than blue light. (The glass slows down the blue light more than it does the red and so the direction of the blue is changed more.)

Now, if white light is used the prism splits up the light into a series of colours. This shows that white light is actually made up of many other colours - a fact first shown by Newton in 1666. The spread of colour is called a SPECTRUM.

The dispersion of white light into a spectrum occurs because the different colours are refracted by different amounts by the glass of the prism.

Violet light is refracted most by a prism and red light is refracted least

The colours of the spectrum are:

although there is really no sharp break between one colour and the next.

A pure spectrum can be produced by adding two lenses to focus each colour to a point on the screen. If this is not done the colours will overlap.

A rainbow

A rainbow is formed in the sky by white light from the sun being refracted by water droplets in the air. The colours are split up with the red on the outside. To see a rainbow you must be facing away from the sun towards the rainstorm.

The photograph shows part of a rainbow that I saw over Lake Windermere in Cumbria. You can see not only the rainbow itself but also its reflection in the lake. If you look very carefully you can see part of the double rainbow, made by the light reflecting inside the raindrops to make this.


schoolphysics prism refraction animation

To see an animation of the refraction of monochromatic light by a prism click on the animation link here.

schoolphysics prism refraction of light of different colours animation

To see an animation of the refraction of light of different colours by a prism click on the animation link here.

Adding colours

You can do the reverse of this experiment by adding colours together. The simplest way to do this is by spinning a coloured disc on which are painted the colours of the spectrum.

The result will be something like white! (a sample disc is shown in the diagram in reality there are more colours and they are not all the same width, this is because of the different sensitivity of your eyes to different colours).

Adding coloured lights

You probably know that you can make different colours by mixing paints together, well the same is true in Physics using light.

You would find that if you have three lights, one red, one blue and one green you could make any other colour by using different combinations and brightnesses of these three.

For this reason red, green and blue are called
The primary colours in Physics are red, blue and green.
The diagram at the side shows the result of adding different combinations, notice that if you add all three together you see white.
Any two colours that can be added together to make white are called COMPLEMENTARY COLOURS.

How things look in different coloured lights

The colour of an object can look very different if the colour of the light shining on it is changed. This is because a surface will only reflect certain colours. This is especially important when choosing paints or clothes in a shop where there are coloured lights. You should always look at things in daylight (white light) to see their true colours. The results of shining different colours of light on different coloured surfaces is shown below.

Appearance in different colours

White light Red light Blue light Green light
Red surface Red Black Black
Blue surface Black Blue Black
Green surface Black Black Green
© Keith Gibbs 2020