In the previous section we saw recombination of colours. We also saw persistence of vision. In this section, we will see primary, secondary and complementary colours.
Let us do an activity. We will write it in steps:
1. Take three torches. Cover the front ends with red, blue and green glass papers.
2. Take the red and blue torches.
• Turn them on and direct the lights towards a white wall.
• We can see a red circle and a blue circle. This is shown in fig.13.8(a) below:
3. Now move the torches in such a way that, the red circle and blue circle overlap.
This is shown in fig.13.8(b) above.
• When they overlap, there is a 'region common to both circles'.
• This common region is magenta in colour.
4. Take the red and green torches.
• Turn them on and direct the lights towards the white wall.
• We can see a red circle and a green circle. This is shown in fig.13.8(c) above.
5. Now move the torches in such a way that, the red circle and green circle overlap.
This is shown in fig.13.8(d) above.
• When they overlap, there is a 'region common to both circles'.
• This common region is yellow in colour.
6. Take the blue and green torches.
• Turn them on and direct the lights towards the white wall.
• We can see a blue circle and a green circle. This is shown in fig.13.9(a) below:
7. Now move the torches in such a way that, the blue circle and green circle overlap.
This is shown in fig.13.9(b)
• When they overlap, there is a 'region common to both circles'.
• This common region is cyan in colour.
8. Now take all the three torches together.
• Turn them on and direct the lights towards the white wall
• Move the torches in such a way that the the three circles overlap. This is shown in fig.13.9(d)
♦ We can see a common region between red and blue. It is magenta as before
♦ We can see a common region between red and green. It is yellow as before
♦ We can see a common region between green and blue. It is cyan as before
• But there is something new this time:
■ The region which is common to all colours is white
■ So we can write:
White light can be obtained by combining red, blue and green colours.
9. Remove the red blue and green glass papers from the torch
• Cover them with other coloured glass papers.
• Now try to make new colours by different combinations.
• After some trials, we will find that, no matter how many ways we try, it is not possible to make red, blue or green.
■ By suitably superimposing red, blue and green, it is possible to make white light or other colours.
■ But it is impossible to make red, blue and green by combining other colours.
■ Hence red, blue and green are the primary colours of light.
■ The colour obtained by combining any two primary colours of the same intensity is a secondary colour of light.
Note that, 'same intensity' is specially mentioned.
From the above activity we can write the following 3 points about secondary colours:
(i) When the primary colours blue and red are combined we get the secondary colour magenta
(ii) When the primary colours green and red are combined we get the secondary colour yellow
(iii) When the primary colours blue and green are combined we get the secondary colour cyan
■ An easy method to remember primary and secondary colours:
Step 1: Remember the two combinations 'RGB' and 'CMY'
'RGB' stands for the primary colours red, green and blue
'CMY' stands for the secondary colours cyan, magenta and yellow
Step 2: Write them vertically downwards, side by side as shown in fig. 13.10 (a) below:
• R must correspond to C
• G must correspond to M
• B must correspond to Y
Step 3: If we want to know how C is obtained:
♦ Strike out the primary colour 'R' corresponding to C
♦ Then C is obtained by the other two primary colours G and B
♦ This is shown in fig.13.10(b) above
• If we want to know how M is obtained:
♦ Strike out the primary colour 'G' corresponding to M
♦ Then M is obtained by the other two primary colours R and B
♦ This is shown in fig.13.10(c) above
• If we want to know how Y is obtained:
♦ Strike out the primary colour 'B' corresponding to Y
♦ Then Y is obtained by the other two primary colours R and G
♦ This is shown in fig.13.10(d) above
From the previous activity, we already have three torches: red, blue and green
1. Take three more torches. Cover the front ends with the secondary colours: cyan, magenta and yellow glass papers.
2. Take the red and cyan torches.
• Turn them on and direct the lights towards the white wall.
• We can see a red circle and a cyan circle. This is shown in fig.13.11(a) below:
3. Now move the torches in such a way that, the red circle and cyan circle overlap.
This is shown in fig.13.11(b) above.
• When they overlap, there is a 'region common to both circles'.
• We can see that this common region is white in colour.
4. Take the Green and magenta torches.
• Turn them on and direct the lights towards the white wall.
• We can see a green circle and a magenta circle. This is shown in fig.13.11(c) above
5. Now move the torches in such a way that, the green circle and magenta circle overlap.
This is shown in fig.13.11(d) above.
• When they overlap, there is a 'region common to both circles'.
• We can see that this common region is white in colour.
6. Take the blue and yellow torches.
• Turn them on and direct the lights towards the white wall.
• We can see a blue circle and a yellow circle. This is shown in fig.13.11(e) above
7. Now move the torches in such a way that, the blue circle and yellow circle overlap.
This is shown in fig.13.11(f) above.
• When they overlap, there is a 'region common to both circles'.
• We can see that this common region is white in colour.
• From the above activity, we get three new methods to obtain white colour.
• In each of those methods, only two colours are mixed.
• Note that, in the previous activity be mixed three colours together to obtain white light.
■ If white light is formed by combining just two colours, each one of those two colour is said to be complementary to the other.
case 1:
(i) We mixed red and cyan together. Red is a primary colour while cyan is a secondary colour.
(ii) When cyan is included, we are automatically including blue and green.
(iii) So in effect, we are mixing red, blue and green together. Thus we get white colour.
case 2:
(i) We mixed green and magenta together. Green is a primary colour while magenta is a secondary colour.
(ii) When magenta is included, we are automatically including red and blue.
(iii) So in effect, we are mixing red, blue and green together. Thus we get white colour.
case 3:
(i) We mixed blue and yellow together. Blue is a primary colour while yellow is a secondary colour.
(ii) When yellow is included, we are automatically including red and green.
(iii) So in effect, we are mixing red, blue and green together. Thus we get white colour.
If white light is formed by combining a primary colour and secondary colour, two are mutually complementary.
■ An easy method to remember complementary colours:
Step 1: Remember the two combinations 'RGB' and 'CMY'
'RGB' stands for the primary colours red, green and blue
'CMY' stands for the secondary colours cyan, magenta and yellow
Step 2: Write them vertically downwards, side by side as shown in fig. 13.10 (a) above.
• That is it. We have three pairs: [R,C], [G,Y], [B,M]
• Consider any one pair. Each member of that pair will be the complement colour of the other member
Let us do an activity. We will write it in steps:
1. Take three torches. Cover the front ends with red, blue and green glass papers.
2. Take the red and blue torches.
• Turn them on and direct the lights towards a white wall.
• We can see a red circle and a blue circle. This is shown in fig.13.8(a) below:
 |
| Fig.13.8 |
This is shown in fig.13.8(b) above.
• When they overlap, there is a 'region common to both circles'.
• This common region is magenta in colour.
4. Take the red and green torches.
• Turn them on and direct the lights towards the white wall.
• We can see a red circle and a green circle. This is shown in fig.13.8(c) above.
5. Now move the torches in such a way that, the red circle and green circle overlap.
This is shown in fig.13.8(d) above.
• When they overlap, there is a 'region common to both circles'.
• This common region is yellow in colour.
6. Take the blue and green torches.
• Turn them on and direct the lights towards the white wall.
• We can see a blue circle and a green circle. This is shown in fig.13.9(a) below:
 |
| Fig.13.9 |
This is shown in fig.13.9(b)
• When they overlap, there is a 'region common to both circles'.
• This common region is cyan in colour.
8. Now take all the three torches together.
• Turn them on and direct the lights towards the white wall
• Move the torches in such a way that the the three circles overlap. This is shown in fig.13.9(d)
♦ We can see a common region between red and blue. It is magenta as before
♦ We can see a common region between red and green. It is yellow as before
♦ We can see a common region between green and blue. It is cyan as before
• But there is something new this time:
■ The region which is common to all colours is white
■ So we can write:
White light can be obtained by combining red, blue and green colours.
9. Remove the red blue and green glass papers from the torch
• Cover them with other coloured glass papers.
• Now try to make new colours by different combinations.
• After some trials, we will find that, no matter how many ways we try, it is not possible to make red, blue or green.
■ By suitably superimposing red, blue and green, it is possible to make white light or other colours.
■ But it is impossible to make red, blue and green by combining other colours.
■ Hence red, blue and green are the primary colours of light.
■ The colour obtained by combining any two primary colours of the same intensity is a secondary colour of light.
Note that, 'same intensity' is specially mentioned.
From the above activity we can write the following 3 points about secondary colours:
(i) When the primary colours blue and red are combined we get the secondary colour magenta
(ii) When the primary colours green and red are combined we get the secondary colour yellow
(iii) When the primary colours blue and green are combined we get the secondary colour cyan
■ An easy method to remember primary and secondary colours:
Step 1: Remember the two combinations 'RGB' and 'CMY'
'RGB' stands for the primary colours red, green and blue
'CMY' stands for the secondary colours cyan, magenta and yellow
Step 2: Write them vertically downwards, side by side as shown in fig. 13.10 (a) below:
 |
| Fig.13.10 |
• G must correspond to M
• B must correspond to Y
Step 3: If we want to know how C is obtained:
♦ Strike out the primary colour 'R' corresponding to C
♦ Then C is obtained by the other two primary colours G and B
♦ This is shown in fig.13.10(b) above
• If we want to know how M is obtained:
♦ Strike out the primary colour 'G' corresponding to M
♦ Then M is obtained by the other two primary colours R and B
♦ This is shown in fig.13.10(c) above
• If we want to know how Y is obtained:
♦ Strike out the primary colour 'B' corresponding to Y
♦ Then Y is obtained by the other two primary colours R and G
♦ This is shown in fig.13.10(d) above
Complementary colours
Let us do another activity:From the previous activity, we already have three torches: red, blue and green
1. Take three more torches. Cover the front ends with the secondary colours: cyan, magenta and yellow glass papers.
2. Take the red and cyan torches.
• Turn them on and direct the lights towards the white wall.
• We can see a red circle and a cyan circle. This is shown in fig.13.11(a) below:
 |
| Fig.13.11 |
This is shown in fig.13.11(b) above.
• When they overlap, there is a 'region common to both circles'.
• We can see that this common region is white in colour.
4. Take the Green and magenta torches.
• Turn them on and direct the lights towards the white wall.
• We can see a green circle and a magenta circle. This is shown in fig.13.11(c) above
5. Now move the torches in such a way that, the green circle and magenta circle overlap.
This is shown in fig.13.11(d) above.
• When they overlap, there is a 'region common to both circles'.
• We can see that this common region is white in colour.
6. Take the blue and yellow torches.
• Turn them on and direct the lights towards the white wall.
• We can see a blue circle and a yellow circle. This is shown in fig.13.11(e) above
7. Now move the torches in such a way that, the blue circle and yellow circle overlap.
This is shown in fig.13.11(f) above.
• When they overlap, there is a 'region common to both circles'.
• We can see that this common region is white in colour.
• From the above activity, we get three new methods to obtain white colour.
• In each of those methods, only two colours are mixed.
• Note that, in the previous activity be mixed three colours together to obtain white light.
■ If white light is formed by combining just two colours, each one of those two colour is said to be complementary to the other.
case 1:
(i) We mixed red and cyan together. Red is a primary colour while cyan is a secondary colour.
(ii) When cyan is included, we are automatically including blue and green.
(iii) So in effect, we are mixing red, blue and green together. Thus we get white colour.
case 2:
(i) We mixed green and magenta together. Green is a primary colour while magenta is a secondary colour.
(ii) When magenta is included, we are automatically including red and blue.
(iii) So in effect, we are mixing red, blue and green together. Thus we get white colour.
case 3:
(i) We mixed blue and yellow together. Blue is a primary colour while yellow is a secondary colour.
(ii) When yellow is included, we are automatically including red and green.
(iii) So in effect, we are mixing red, blue and green together. Thus we get white colour.
If white light is formed by combining a primary colour and secondary colour, two are mutually complementary.
■ An easy method to remember complementary colours:
Step 1: Remember the two combinations 'RGB' and 'CMY'
'RGB' stands for the primary colours red, green and blue
'CMY' stands for the secondary colours cyan, magenta and yellow
Step 2: Write them vertically downwards, side by side as shown in fig. 13.10 (a) above.
• That is it. We have three pairs: [R,C], [G,Y], [B,M]
• Consider any one pair. Each member of that pair will be the complement colour of the other member
In the next section, we will see colour of opaque objects.
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