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6 Light
6.4 Colours of light
In this topic you will:
• discover what happens when colours of light are added
• discover what happens when colours of light are subtracted
• discover why we see different colours.
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Getting started Key words
Work in pairs. absorbed
coloured filters
other groups?
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Make a list of all the colours you can see in this picture of flowers.
How many did you get? How does this number compare with
cyan
magenta
non-luminous
primary colours
subtraction
transmit
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6.4 Colours of light
Primary colours
Topic 3.3 explained 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:
• red • green • blue.
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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
•
•
•
red + blue = magenta
blue + green = cyan
red + green + blue = white.
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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
magenta
blue
red
white
cyan
yellow
green
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The different colours that you see on a mobile phone, computer or Colours formed by overlapping the
television are all produced from combinations of the three primary three primary colours of light
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.
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The colours on this phone display are made by adding the Close-up of a television screen showing the sources of the
three primary colours of light. primary colours of light.
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6 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
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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
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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.
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When these seven colours arrive at the red filter, only red is
transmitted. The other six are absorbed. This is shown in These traffic lights use coloured filters.
the diagram.
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Figure 6.4.3: A coloured filter works by absorbing the colours of light that are different from
the colour of the filter.
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.
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6.4 Colours of light
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.
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PL
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This stage light is using a white lamp and a blue filter. What colour was the filter used in taking this photograph?
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
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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. Green is one of the colours that a red filter absorbs.
white light only green no green light
light passes passes through
through
green filter red filter
The result of using two different coloured filters
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6 Light
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.
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white light coming in
PL yellow light
yellow surface
The flower appears yellow because it reflects yellow light and absorbs the other colours.
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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 which colours
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of light they reflect and which they absorb.
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6.4 Colours of light
Look at the two cubes in Figure 6.4.7. One is red and A
the other is blue. The cubes are shown in different blue and red
colours of light. cubes seen
in white light
The red cube appears black when seen in green light.
This is because red objects only reflect red light.
B
They absorb all other colours. Here, the red cube is blue and red
absorbing the blue light and not reflecting any light. cubes seen
in blue light
The blue cube appears black when seen in red light for
the same reason. It absorbs the red light and does not
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reflect any light. C
blue and red
Questions cubes seen
in red light
1 List the three primary colours of light.
2
3
Name the colour produced when
a
b
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red light and blue light are added together
red light and green light are added together
red, green and blue lights are added together.
A stage light uses a white lamp.
What colour of light will be seen when:
a a yellow filter is used
a blue and red cubes in white light; b the same cubes
in green light; c the same cubes in red light.
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b an orange filter is used.
4 a A green ball appears green. What colour of light could be
shining on the green ball? Choose two.
blue green red white magenta
b A blue ball appears black. What two colours of light could be
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shining on the blue ball?
blue green red white
5 A T-shirt looks red. What could explain this?
Write three letters.
A the T-shirt is red and is seen in white light
B the T-shirt is red and is seen in red light
C the T-shirt is blue and is seen in green light
D the T-shirt is white and is seen in red light
E the T-shirt is yellow and is seen in blue light
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