In the previous section we saw the formation of rainbow. In this section, we will see recombination of colours.
Consider figure 13.6 below:
1. White light is falling on the prism on the left.
• As a result, dispersion occurs.
• Seven colours emerged from this prism.
2. These 7 colours fall on the surface of the prism on the right.
• This prism is similar to the first prism. But it is kept with its base on the top.
• We can see that the light emerging from the second prism is a single beam of white light.
3. So what happens inside the second prism?
Ans: Each colour emerging from the first prism is bent by the glass material of the second prism.
• This 'bending' occurs in a direction opposite to 'the bending that occurred in the first prism'
• The result is that, the seven rays converge at a point on the opposite face of the second prism.
4. When the seven colours converge at a point, we get white light.
■ This experiment proves that, if the seven colours are combined together, we will get white light.
• Seven colours of the rainbow are painted on it.
2. The area of each colour in the disk should be the same.
■ How can we achieve the same area?
Ans:
(i) We know that, the full circle has a central angle of 360o.
(ii) If we divide this 360 by 7, we will get 51.42.
• So the central angle of each sector on the disk is 51.42o
(iv) We have learned about sectors in our maths classes. (Details here)
3. Once we divide the circle into 7 equal sectors, we can give each sector a colour.
• The colouring should be done in the same order as in the rainbow. That is., the order in the word VIBGYOR
• Such a disc is called Newton's colour disc.
4. If we spin this disc about it's centre, we can see that the colours gradually fade.
• If we spin it very fast, we will see only white colour.
5. We want to know the reason for obtaining white.
We will write it in steps:
(i) Consider any one colour in the disc, say blue.
(ii) Consider the instant at which the ray from the blue colour falls on our eye.
(iii) Just after that instant, the ray from the next colour green will reach the eye. This is because, the disc is spinning fast
(iii) Even when the green colour has reached the eye, the previous blue ray will be still remaining in the retina
■ This is because, a ray will remain in the retina for (1⁄16) seconds. This is called persistence of vision
(iv) In fact, if the wheel spins very fast, the rays from all 7 colours will reach the retina within (1⁄16) seconds
(v) Since all the rays are together at a point within such a small interval of time, we will not be able to distinguish between the various colours
(vi) We will see only the resulting white colour
■ This proves that, white colour is a combination of seven colours.
Some examples of persistence of vision:
• When a lighted torch is rotated fast, it appears as an illuminated circle
• Continuously falling rain drops appear as glass rods
• The leaves of a rotating fan cannot be distinguished from one another
Consider figure 13.6 below:
 |
| Fig.13.6 |
• As a result, dispersion occurs.
• Seven colours emerged from this prism.
2. These 7 colours fall on the surface of the prism on the right.
• This prism is similar to the first prism. But it is kept with its base on the top.
• We can see that the light emerging from the second prism is a single beam of white light.
3. So what happens inside the second prism?
Ans: Each colour emerging from the first prism is bent by the glass material of the second prism.
• This 'bending' occurs in a direction opposite to 'the bending that occurred in the first prism'
• The result is that, the seven rays converge at a point on the opposite face of the second prism.
4. When the seven colours converge at a point, we get white light.
■ This experiment proves that, if the seven colours are combined together, we will get white light.
Newtons' Colour Disc
1. Consider the disk shown in fig.13.7 below. |
| Fig.13.7 |
2. The area of each colour in the disk should be the same.
■ How can we achieve the same area?
Ans:
(i) We know that, the full circle has a central angle of 360o.
(ii) If we divide this 360 by 7, we will get 51.42.
• So the central angle of each sector on the disk is 51.42o
(iv) We have learned about sectors in our maths classes. (Details here)
3. Once we divide the circle into 7 equal sectors, we can give each sector a colour.
• The colouring should be done in the same order as in the rainbow. That is., the order in the word VIBGYOR
• Such a disc is called Newton's colour disc.
4. If we spin this disc about it's centre, we can see that the colours gradually fade.
• If we spin it very fast, we will see only white colour.
5. We want to know the reason for obtaining white.
We will write it in steps:
(i) Consider any one colour in the disc, say blue.
(ii) Consider the instant at which the ray from the blue colour falls on our eye.
(iii) Just after that instant, the ray from the next colour green will reach the eye. This is because, the disc is spinning fast
(iii) Even when the green colour has reached the eye, the previous blue ray will be still remaining in the retina
■ This is because, a ray will remain in the retina for (1⁄16) seconds. This is called persistence of vision
(iv) In fact, if the wheel spins very fast, the rays from all 7 colours will reach the retina within (1⁄16) seconds
(v) Since all the rays are together at a point within such a small interval of time, we will not be able to distinguish between the various colours
(vi) We will see only the resulting white colour
■ This proves that, white colour is a combination of seven colours.
Some examples of persistence of vision:
• When a lighted torch is rotated fast, it appears as an illuminated circle
• Continuously falling rain drops appear as glass rods
• The leaves of a rotating fan cannot be distinguished from one another
In the next section, we will see primary and secondary colours.
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