Making rainbows
In this topic you will:
- learn how white light is made from many colours
- discover how dispersion of white light can be done with a prism
- be able to recall the colours of white light in the correct order.
Key words
- dispersion
- prism
- spectrum
- triangular
Newton’s discovery about light
The name Newton is often associated with forces, but Isaac Newton made many important discoveries.
In the year 1666, Newton showed that white light could be split into different colours. The picture shows Newton using light from the Sun, coming through a hole. Newton is using a glass block in the shape of a triangle to split the light into its colours. He is shining the colours onto a screen.
The next picture shows what the colours look like when seen on a screen. These are the colours that make up white light.
The spectrum
The range of colours that can be seen in white light is called a spectrum. In the spectrum, the colours are not separate but they merge from one to another.
Some people think Newton originally saw five or six colours, but most people now agree that there are seven. In the order that they appear in the spectrum, these seven colours are:
redorangeyellowgreenblueindigoviolet
You can remember the order of the colours using a made-up person’s name: ‘ROY G BIV’.
Dispersion
Dispersion means splitting light into different colours. Dispersion happens because light is refracted. Each of the different colours of light that make up white light is refracted through a slightly different angle. This can be shown by using a triangular prism. Triangular means in the shape of a triangle. A prism is a solid shape such as the one that Newton used.
When a ray of white light passes through the prism, the ray is refracted. Violet light is refracted through the largest angle and red light is refracted through the smallest angle. You can see this in the picture.
Quick fact
Isaac Newton used a triangular glass prism to show that white light is made up of different colours. This discovery helped build the foundation for modern optics.
Rainbows
A rainbow is formed when drops of water in the air cause dispersion of light. The light is also reflected from inside the drops of water. That means that, for you to see a rainbow:
- the Sun must be shining, to provide bright light
- there must be rain or small drops of water in the air, to cause dispersion of light
- the Sun must be behind you, because the water drops reflect the light inside them.
The drops of water from the hosepipe in the photograph are making a rainbow. The Sun is shining and the Sun is behind the camera.
Quick fact
Rainbows are always seen with the Sun behind the observer and rain or mist in front. The light reflects and refracts inside water droplets to form the visible spectrum.
Questions
Show Answer
Dispersion
What name describes this piece of equipment?
Write one letter.
A. round glass cylinder
B. square glass prism
C. triangular glass prism
D. hexagonal glass prism
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C
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A
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A
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Spectrum
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Drops of water are needed to refract and reflect the light, which causes the separation of white light into different colours — forming a rainbow.
How many colours will these people say they can see in the rainbow?
Show Answer
Six
Think Like a Scientist
Making a rainbow
In this activity, you will change variables and describe how observations change. You will work independently.
You will need:
- ray box and suitable power supply
- triangular glass prism
- piece of white paper or card to use as a screen
Safety:
Do not put your eye closer to the prism than about one metre. The light will become very bright and could cause damage to your eye.
Make the room as dark as possible. Set up the equipment as shown in the diagram.
Method: Part 1
- Adjust the positions of the ray box and the screen until you see the colours of the rainbow on the screen.
- Move the screen closer to the prism.
- Move the screen further away from the prism.
Questions
Show Answer
Usually 5 to 7 colours are visible, depending on the observer and setup.
Show Answer
Red, orange, yellow, green, blue, indigo, violet.
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Red.
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Violet.
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The colours appeared closer together and less spread out. They may have been harder to distinguish.
Show Answer
The colours spread out more and became easier to see clearly.
Method: Part 2
- Remove the screen and stand about 1 metre from the prism, in the same direction as the screen had been.
- Move so that you are looking into the refracted rays with one eye.
- Move from side to side to see different colours.
- Increase the distance between your eye and the prism to about 2 metres.
- Again, move side to side to observe the colours.
Questions
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The colours may become fainter but more separated; viewing angles become wider.
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Advantage: You can clearly record what you see on the screen. Disadvantage: The screen may limit viewing angles or resolution of colours.
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Strength: Both involve dispersion of white light into colours. Limitation: Rainbows form from raindrops, not prisms.
Self-assessment
Different people see different numbers of colours in this activity. The numbers usually vary from 5 to 7.
Did you see the same number as everyone else in the class?
Suggest reasons why people see different numbers of colours.