In the previous section we saw colour of transparent objects. In this section, we will see the electromagnetic spectrum. We will write the details in steps:
1. We have seen that, the white light is composed of seven colours:
Violet, Indigo, Blue, Green, Yellow, Orange and Red.
2. All these seven coloured lights, travel in the form of waves
3. We have seen some basics about wave motion (Details here)
• The distance between any to consecutive crests is the wave length
4. Consider the red light. The distance between any two consecutive crests of it's wave is approximately 700 nm.
♦ 'nm' stands for nanometer
♦ One nanometer is (1× 10-9) metre
• So we can see that wave length the red light is very small
5. Similarly, wave length of violet is approximately 400 nm
Let us plot these values on a graph. It is shown in fig.13.19(a) below:
• 700 nm is marked on the x axis. At that point red colour is shown
• 400 nm is marked on the x axis. At that point violet colour is shown
6. Normally, as we move from left to right on the x axis, the values increase
• But here, the values decreases. Let us see the reason:
• We know that, wavelength is inversely proportional to frequency
• That is., when wave length decreases, frequency increases.
• So if we consider frequency, the values indeed increase when we move from left to right along the x axis in fig.13.19(a) above
7. The other five colours can be arranged in order between red and violet.
• This is shown in fig.b above
8. Now two questions arise:
(i) Is there any wave which has 'wave length greater than 700 nm'?
(ii) Is there any wave which has 'wave length less than 400 nm'?
9. The answers:
(i) There are indeed waves which have 'wave length greater than 700 nm'
• But human eyes cannot see 'waves with wave lengths greater than 700 nm'
♦ In other words they are 'invisible waves'
• 'Waves with wave lengths ranging from 1 millimetre to 700 nm' are called infrared waves
• Since those wave lengths are greater than 700 nm, we plot them on the left side of red
• This is shown in fig.13.20(a) below.
(ii) There are indeed waves which have 'wave length less than 400 nm'
• But human eyes cannot see 'waves with wave lengths less than 400 nm'
♦ In other words, just like infra red, these are also 'invisible waves'
• 'Waves with wave lengths ranging from 400 nm to 1 nm' are called ultraviolet waves
• Since those wave lengths are less than 400 nm, we plot them on the right side of violet
• This is shown in fig.13.20(a) below:
10. We know that white light comes from the sun
• The two new waves that we saw (infrared and ultraviolet) also come from the sun
• So we have a group consisting of three items:
(i) infrared waves
(ii) White light waves
(iii) ultraviolet waves
11. This group is given a special name: solar spectrum
12 If we allow the 'waves from the sun' to pass through a prism, they will undergo refraction
♦ The components will thus get separated from each other
• But we are not able to see the infrared and ultraviolet waves
• We can see only the seven colours
• So these seven colours together is known as visible spectrum
13. In the fig.13.20(a), the seven colours are shown distinct from each other. This is for a better understanding only.
• In the actual case, there is a 'gradual gradation' from one colour to the next. An example is shown in fig.13.20(b).
• The red colour at the extreme left gradually merges into the next colour orange.
• We can see that the red colour is fading as we move to the right.
• As we move to the right, it becomes more and more orange.
• This indicates that, the wavelength of the red light is not the same everywhere.
• As we move to the right, the wavelength also decreases.
• Indeed experiments indicate that wavelength of red light varies from 700 to 620 nm
■ Some features of infrared waves:
• When objects are heated, the molecules in them vibrate. Those vibrating molecules emit heat in the form of infrared waves. We cannot see those waves. But we can feel it
• The heat from the sun reaches us in the form of infrared waves
• Infrared waves are used in remote controls and night vision cameras
■ Some features of visible light:
• Causes sense of vision
• Helps to produce energy by photosynthesis
• Used in solar cells
• It can also affect our vision.
• The UV rays from the sun is absorbed by the ozone layer. So most of the UV rays do not reach the earth.
• UV rays in suitable doses can help to produce vitamin D in the skin
14. We have seen that the visible spectrum is a part of the solar spectrum. This is clear from fig.13.20(a) above.
• Now two questions arise:
(i) Is there any wave which has 'wave length greater than 1 mm'?
(ii) Is there any wave which has 'wave length less than 1 nm'?
15. The answers:
(i) There are indeed waves which have 'wave length greater than 1 mm'
• 'Waves with wave lengths ranging from 0.1 m to 1 mm' are called microwaves
• 'Waves with wave lengths greater than 0.1 m are called radiowaves
• Since those wave lengths are greater than 1 mm, we plot them on the left side of infrared
• This is shown in fig.13.21 below.
(ii) There are indeed waves which have 'wave length less than 1 nm'
• 'Waves with wave lengths ranging from 1 nm to 10-3 nm' are called X-rays
• 'Waves with wave lengths less than 10-3 nm are called Gamma rays
• Since those wave lengths are less than 1 nm, we plot them on the right side of ultraviolet
• This is shown in fig.13.21 below:
■ Some features of radio waves:
• Used for very high frequency radio transmission
• Used for Ultra high frequency television transmission
■ Some features of microwaves:
• Used in radar and mobile phone
• Used in microwave oven
■ Some features of x-rays:
• Penetrate through flesh
• Helps to detect defects of bones breakage of pipes in industries etc
• Dissociates DNA . Hence excessive exposure causes cancer
■ Some features of gamma rays:
• Causes damage to living cells
• Comes out in large quantities during nuclear fission
• Used in cancer treatment
• Useful in sterilizing surgical instruments
■ The waves ranging from radio waves to gamma rays is called electromagnetic spectrum
1. We have seen that, the white light is composed of seven colours:
Violet, Indigo, Blue, Green, Yellow, Orange and Red.
2. All these seven coloured lights, travel in the form of waves
3. We have seen some basics about wave motion (Details here)
• The distance between any to consecutive crests is the wave length
4. Consider the red light. The distance between any two consecutive crests of it's wave is approximately 700 nm.
♦ 'nm' stands for nanometer
♦ One nanometer is (1× 10-9) metre
• So we can see that wave length the red light is very small
5. Similarly, wave length of violet is approximately 400 nm
Let us plot these values on a graph. It is shown in fig.13.19(a) below:
 |
| Fig.13.19 |
• 400 nm is marked on the x axis. At that point violet colour is shown
6. Normally, as we move from left to right on the x axis, the values increase
• But here, the values decreases. Let us see the reason:
• We know that, wavelength is inversely proportional to frequency
• That is., when wave length decreases, frequency increases.
• So if we consider frequency, the values indeed increase when we move from left to right along the x axis in fig.13.19(a) above
7. The other five colours can be arranged in order between red and violet.
• This is shown in fig.b above
8. Now two questions arise:
(i) Is there any wave which has 'wave length greater than 700 nm'?
(ii) Is there any wave which has 'wave length less than 400 nm'?
9. The answers:
(i) There are indeed waves which have 'wave length greater than 700 nm'
• But human eyes cannot see 'waves with wave lengths greater than 700 nm'
♦ In other words they are 'invisible waves'
• 'Waves with wave lengths ranging from 1 millimetre to 700 nm' are called infrared waves
• Since those wave lengths are greater than 700 nm, we plot them on the left side of red
• This is shown in fig.13.20(a) below.
(ii) There are indeed waves which have 'wave length less than 400 nm'
• But human eyes cannot see 'waves with wave lengths less than 400 nm'
♦ In other words, just like infra red, these are also 'invisible waves'
• 'Waves with wave lengths ranging from 400 nm to 1 nm' are called ultraviolet waves
• Since those wave lengths are less than 400 nm, we plot them on the right side of violet
• This is shown in fig.13.20(a) below:
 |
| Fig.13.20 |
• The two new waves that we saw (infrared and ultraviolet) also come from the sun
• So we have a group consisting of three items:
(i) infrared waves
(ii) White light waves
(iii) ultraviolet waves
11. This group is given a special name: solar spectrum
12 If we allow the 'waves from the sun' to pass through a prism, they will undergo refraction
♦ The components will thus get separated from each other
• But we are not able to see the infrared and ultraviolet waves
• We can see only the seven colours
• So these seven colours together is known as visible spectrum
13. In the fig.13.20(a), the seven colours are shown distinct from each other. This is for a better understanding only.
• In the actual case, there is a 'gradual gradation' from one colour to the next. An example is shown in fig.13.20(b).
• The red colour at the extreme left gradually merges into the next colour orange.
• We can see that the red colour is fading as we move to the right.
• As we move to the right, it becomes more and more orange.
• This indicates that, the wavelength of the red light is not the same everywhere.
• As we move to the right, the wavelength also decreases.
• Indeed experiments indicate that wavelength of red light varies from 700 to 620 nm
■ Some features of infrared waves:
• When objects are heated, the molecules in them vibrate. Those vibrating molecules emit heat in the form of infrared waves. We cannot see those waves. But we can feel it
• The heat from the sun reaches us in the form of infrared waves
• Infrared waves are used in remote controls and night vision cameras
■ Some features of visible light:
• Causes sense of vision
• Helps to produce energy by photosynthesis
• Used in solar cells
■ Some features of ultraviolet waves:
• Exposure to UV rays can cause skin cancer. • It can also affect our vision.
• The UV rays from the sun is absorbed by the ozone layer. So most of the UV rays do not reach the earth.
• UV rays in suitable doses can help to produce vitamin D in the skin
14. We have seen that the visible spectrum is a part of the solar spectrum. This is clear from fig.13.20(a) above.
• Now two questions arise:
(i) Is there any wave which has 'wave length greater than 1 mm'?
(ii) Is there any wave which has 'wave length less than 1 nm'?
15. The answers:
(i) There are indeed waves which have 'wave length greater than 1 mm'
• 'Waves with wave lengths ranging from 0.1 m to 1 mm' are called microwaves
• 'Waves with wave lengths greater than 0.1 m are called radiowaves
• Since those wave lengths are greater than 1 mm, we plot them on the left side of infrared
• This is shown in fig.13.21 below.
(ii) There are indeed waves which have 'wave length less than 1 nm'
• 'Waves with wave lengths ranging from 1 nm to 10-3 nm' are called X-rays
• 'Waves with wave lengths less than 10-3 nm are called Gamma rays
• Since those wave lengths are less than 1 nm, we plot them on the right side of ultraviolet
• This is shown in fig.13.21 below:
 |
| Fig.13.21 |
• Used for very high frequency radio transmission
• Used for Ultra high frequency television transmission
■ Some features of microwaves:
• Used in radar and mobile phone
• Used in microwave oven
■ Some features of x-rays:
• Penetrate through flesh
• Helps to detect defects of bones breakage of pipes in industries etc
• Dissociates DNA . Hence excessive exposure causes cancer
■ Some features of gamma rays:
• Causes damage to living cells
• Comes out in large quantities during nuclear fission
• Used in cancer treatment
• Useful in sterilizing surgical instruments
■ The waves ranging from radio waves to gamma rays is called electromagnetic spectrum
In the next section, we will see scattering of light.
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