In the previous two sections we we have seen the following cases:
• Light falling on opaque objects
• Light falling on transparent objects
In this section we will see what happens when light falls on very small particles
Let us do an activity:
In fig.13.22 below, sodium thiosulphate solution (2 g per 50 mL) is taken in a glass container. It is kept on a table in a dark room.
■ Trial 1: Allow white light to fall on one side of the container. This white light will pass through the solution and emerge from the opposite side of the container. This emerging light is focused onto a white screen.
• Observation: We can see the white light on the screen
■ Trial 2: With the light still on, add two drops of hydrochloric acid to the solution in the beaker
• Observation 1: The solution appears sky blue in color
• Observation 2: The focused light on the screen shows a range of colors:
♦ First it is a mixture of yellow, orange and red
♦ Then it is a mixture of orange and red
♦ After that red alone
♦ Finally it becomes black. That is., no light
• Observation 3: In the final stage, when there is no light on the screen, the solution in the beaker is light grey in color
The trials are complete. We will write an analysis:
1. In trial 1, the light was able to pass through the solution without any interruptions
• This is because, the solution was clear. There were no solid particles in it
2. In trial 2, We added hydrochloric acid
• Then a reaction takes place between sodium thiosulphate and hydrochloric acid.
• As a result, some colloidal particles are formed
3. So the light cannot pass as easily as before through the solution
• The bluish components of the white light have shorter wave lengths.
• They will not be able to pass the colloidal particles. Those components are scattered
4. Those scattered components reach our eyes from the solution. They never reach the screen
• So the solution appears blue in color
5. The reddish components have greater wave lengths.
• They are able to bend around the colloidal particles and reach the screen.
♦ So we see a mixture of yellow, orange and red on the screen
6. But as the reaction continues, the colloidal particles become larger and larger
• Because of the increase in size, yellow also cannot pass
♦ So we see a mixture of orange and red on the screen
7. When the size of particles increase further, orange also cannot pass
♦ So we see only red on the screen
8. Finally the sizes become so large that, even red cannot pass.
• At that time, we see no light on the screen
9. At this stage, all the components are scattered at the solution itself.
• So all the components reach our eyes at the same time.
• Because of this mixture of all colors, the solution appears to be pale gray to us
• If we ever want to look at the sun, we must do so only with the help of proper scientific instruments and guidance from experts.
• So when we look at the sky, we must be looking at portions far away from the sun.
• Thus we can say: When we look at the sky, the light that reach our eyes is not the direct light from the sun
• So which light is it?
• Ans: It is the light scattered by the particles in the atmosphere.
• In the above activity, we saw how white light behaves when they pass through small particles
Now consider fig.13.23 below:
We will write an analysis in steps:
1. At noon the sun will be directly above the observer.
• So the light rays have to travel only a short distance.
2. The bluish components of the light which has smaller wave lengths, get scattered easily.
• So the sky appears to be blue.
• This is much like the blue colour (in the initial stage) of the solution in the beaker in the activity that we saw above.
• Note that, in that activity, the size of the particles gradually increases.
♦ So the bluish colour of the solution gradually changes.
♦ But in the atmosphere, there is no change to the size of the particles.
♦ So the blue colour stays
3. The component colours like violet, indigo and blue, which are of shorter wavelength in sunlight, undergo maximum scattering in the atmosphere.
• These colours spread in the atmosphere and the combined effect of these colours is seen as the blue colour of the sky during day time.
4. At sunrise and sunset, the light has to travel very long distances.
• This is also shown in fig.13.23.
• The bluish components of the light gets scattered away. They cannot travel such long distances. So they do not reach the observer.
• But the reddish components which have greater wave lengths can bend around the particles and reach the observer.
• So the sky appears to be reddish at sunrise and sunset
■ On the moon, the sky is dark even during day time.
• This is because, there is no atmosphere for moon. So there are no particles to scatter the light.
• The light from the sun directly reaches the ground surface of the moon. On reaching the surface, the light gets scattered and so we can see the surface of the moon.
• The intensity of scattering depends on the size of particles in the colloid.
• Some times in the early morning hours, we are able to see this effect among the trees. It is shown in the fig. below. The path of sun's rays are visible because, the mist particles get illuminated.
♦ Such films or sensors capture the visible spectrum of light and we get an image
• Infrared cameras use films or sensors which are sensitive to infrared waves
• To prevent the visible light from reaching those special films/sensors, suitable filters are used
• An image is formed on the film/sensors by the infrared waves.
• We get more details of the 'subject which is photographed'
• This is because, infrared waves which have greater wavelength, is less scattered than visible light
• Such detailed photographs are mainly used for scientific research purposes.
Some images can be seen here.
• Light falling on opaque objects
• Light falling on transparent objects
In this section we will see what happens when light falls on very small particles
Let us do an activity:
In fig.13.22 below, sodium thiosulphate solution (2 g per 50 mL) is taken in a glass container. It is kept on a table in a dark room.
 |
| Fig.13.22 |
■ Trial 1: Allow white light to fall on one side of the container. This white light will pass through the solution and emerge from the opposite side of the container. This emerging light is focused onto a white screen.
• Observation: We can see the white light on the screen
■ Trial 2: With the light still on, add two drops of hydrochloric acid to the solution in the beaker
• Observation 1: The solution appears sky blue in color
• Observation 2: The focused light on the screen shows a range of colors:
♦ First it is a mixture of yellow, orange and red
♦ Then it is a mixture of orange and red
♦ After that red alone
♦ Finally it becomes black. That is., no light
• Observation 3: In the final stage, when there is no light on the screen, the solution in the beaker is light grey in color
The trials are complete. We will write an analysis:
1. In trial 1, the light was able to pass through the solution without any interruptions
• This is because, the solution was clear. There were no solid particles in it
2. In trial 2, We added hydrochloric acid
• Then a reaction takes place between sodium thiosulphate and hydrochloric acid.
• As a result, some colloidal particles are formed
3. So the light cannot pass as easily as before through the solution
• The bluish components of the white light have shorter wave lengths.
• They will not be able to pass the colloidal particles. Those components are scattered
4. Those scattered components reach our eyes from the solution. They never reach the screen
• So the solution appears blue in color
5. The reddish components have greater wave lengths.
• They are able to bend around the colloidal particles and reach the screen.
♦ So we see a mixture of yellow, orange and red on the screen
6. But as the reaction continues, the colloidal particles become larger and larger
• Because of the increase in size, yellow also cannot pass
♦ So we see a mixture of orange and red on the screen
7. When the size of particles increase further, orange also cannot pass
♦ So we see only red on the screen
8. Finally the sizes become so large that, even red cannot pass.
• At that time, we see no light on the screen
9. At this stage, all the components are scattered at the solution itself.
• So all the components reach our eyes at the same time.
• Because of this mixture of all colors, the solution appears to be pale gray to us
Colour of the sky
• When we look up into the sky, we must never look at the sun.• If we ever want to look at the sun, we must do so only with the help of proper scientific instruments and guidance from experts.
• So when we look at the sky, we must be looking at portions far away from the sun.
• Thus we can say: When we look at the sky, the light that reach our eyes is not the direct light from the sun
• So which light is it?
• Ans: It is the light scattered by the particles in the atmosphere.
• In the above activity, we saw how white light behaves when they pass through small particles
Now consider fig.13.23 below:
 |
| Fig.13.23 |
1. At noon the sun will be directly above the observer.
• So the light rays have to travel only a short distance.
2. The bluish components of the light which has smaller wave lengths, get scattered easily.
• So the sky appears to be blue.
• This is much like the blue colour (in the initial stage) of the solution in the beaker in the activity that we saw above.
• Note that, in that activity, the size of the particles gradually increases.
♦ So the bluish colour of the solution gradually changes.
♦ But in the atmosphere, there is no change to the size of the particles.
♦ So the blue colour stays
3. The component colours like violet, indigo and blue, which are of shorter wavelength in sunlight, undergo maximum scattering in the atmosphere.
• These colours spread in the atmosphere and the combined effect of these colours is seen as the blue colour of the sky during day time.
4. At sunrise and sunset, the light has to travel very long distances.
• This is also shown in fig.13.23.
• The bluish components of the light gets scattered away. They cannot travel such long distances. So they do not reach the observer.
• But the reddish components which have greater wave lengths can bend around the particles and reach the observer.
• So the sky appears to be reddish at sunrise and sunset
■ On the moon, the sky is dark even during day time.
• This is because, there is no atmosphere for moon. So there are no particles to scatter the light.
• The light from the sun directly reaches the ground surface of the moon. On reaching the surface, the light gets scattered and so we can see the surface of the moon.
Tyndall effect
• When rays of light pass through a colloidal fluid or suspension, the tiny particles get illuminated due to scattering. Because of this, the path of light becomes visible. This is called Tyndall effect.• The intensity of scattering depends on the size of particles in the colloid.
• Some times in the early morning hours, we are able to see this effect among the trees. It is shown in the fig. below. The path of sun's rays are visible because, the mist particles get illuminated.
 |
| Tyndall effect Source: Wikimedia commons |
Infrared photography
• Ordinary cameras use ordinary films or ordinary sensors♦ Such films or sensors capture the visible spectrum of light and we get an image
• Infrared cameras use films or sensors which are sensitive to infrared waves
• To prevent the visible light from reaching those special films/sensors, suitable filters are used
• An image is formed on the film/sensors by the infrared waves.
• We get more details of the 'subject which is photographed'
• This is because, infrared waves which have greater wavelength, is less scattered than visible light
• Such detailed photographs are mainly used for scientific research purposes.
Some images can be seen here.
We have completed this discussion on colours of light. In the next chapter, we will see some basics about electronis.
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