Science 8th grade
UNIT 12: Light 12.5 The spectrum of white light
Science 8th grade
UNIT 12: Light 12.5 The spectrum of white light
Sometimes, if it rains and the Sun shines at the same time, you may see a rainbow. You have to stand with your back to the Sun; you will see an arch of beautiful colours looking as though it is hanging in the air.
Splitting light
You can see the colours of the rainbow for yourself by sending a ray of white light into a glass prism. (A prism is a triangular glass block.)
When the light enters the prism it bends - it is refracted. It also bends as it leaves the prism.
Something else happens. The white light is split up into a spectrum of colours. They are the same colours as you see in a rainbow. The splitting up of white light into separate colours is called dispersion.
The colours of the spectrum always appear in the same order:
red orange yellow green blue indigo violet
(Indigo is a dark blue-purple colour. Although we say there are seven colours in the spectrum, there are no dividing lines where one colour changes to the next. The colour changes gradually from one shade to the next.
Questions
1) Look at the diagram of the spectrum being formed by a prism. Which colour is refracted most as it passes through the prism? Which colour is refracted least?
2) A rainbow appears when sunlight is dispersed to form a spectrum. What is the transparent material that causes this?
3) To remember the colours in the spectrum, some people write this: Roy G.
Biv. It looks like someone's name. How will this help them to remember the order of the colours?
Explaining dispersion
Because a spectrum appears when light passes through a glass prism, some people imagined that it was the glass that gave the colours to the light. Isaac Newton realised that this was incorrect. He showed that white light (such as light from the Sun) is a mixture of all the different colours of the spectrum.
Dispersion happens because of refraction. When white light enters a block of glass, some colours bend more than others. Violet bends the most, red the least. This means that the different colours travel off in different directions, and so we can see them separately.
Question
$A+I$: 4) Which colour of light is refracted more by a prism, green or blue? Explain how you can tell.
Activity 12.5 (A rainbow in the lab)
1) Place the prism on a piece of white paper. Draw around it with a pencil to mark its position.
2) Shine a ray of light from a ray box or a torch into the prism. Aim for a point near one corner of the prism.
3) Adjust the position of the ray until you get a clear spectrum emerging from the prism.
4) On the paper, mark two dots on the ray going into the prism. Mark the positions of the red and violet rays coming from the prism.
5) Remove the prism and the light source.
Draw in the rays and label your diagram.
