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
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 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.
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
Adding coloured lights
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 calledPRIMARY COLOURS
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
How things look in different coloured lights
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.