The earth’s atmosphere is a heterogeneous mixture of minute particles. These particles include smoke, tiny water droplets, suspended particles of dust and molecules of air.
When a beam of light strikes such fine particles, the path of the beam becomes visible. The light reaches us, after being reflected diffusely by these particles.
The phenomenon of scattering of light by the colloidal particles gives rise to Tyndall effect which you have studied in Class IX. This phenomenon is seen when a fine beam of sunlight enters a smoke-filled room through a small hole.
Thus, scattering of light makes the particles visible. Tyndall effect can also be observed when sunlight passes through a canopy of a dense forest. Here, tiny water droplets in the mist scatter light.
The colour of the scattered light depends on the size of the scattering particles. Very fine particles scatter mainly blue light while particles of larger size scatter light of longer wavelengths. If the size of the scattering particles is large enough, then, the scattered light may even appear white.
`ul" Why is the colour of the clear Sky Blue?"`
The molecules of air and other fine particles in the atmosphere have size smaller than the wavelength of visible light. These are more effective in scattering light of shorter wavelengths at the blue end than light of longer wavelengths at the red end.
The red light has a wavelength about 1.8 times greater than blue light. Thus, when sunlight passes through the atmosphere, the fine particles in air scatter the blue colour (shorter wavelengths) more strongly than red.
The scattered blue light enters our eyes. If the earth had no atmosphere, there would not have been any scattering. Then, the sky would have looked dark. The sky appears dark to passengers flying at very high altitudes, as scattering is not prominent at such heights.
You might have observed that ‘danger’ signal lights are red in colour. Do you know why? The red is least scattered by fog or smoke. Therefore, it can be seen in the same colour at a distance.
`ul" Colour of the Sun at Sunrise and Sunset"`
Have you seen the sky and the Sun at sunset or sunrise? Have you wondered as to why the Sun and the surrounding sky appear red?
Let us do an activity to understand the blue colour of the sky and the reddish appearance of the Sun at the sunrise or sunset.
♦ Place a strong source (S) of white light at the focus of a converging lens `(L_1)`. This lens provides a parallel beam of light.
♦ Allow the light beam to pass through a transparent glass tank (T) containing clear water.
♦ Allow the beam of light to pass through a circular hole (c) made in a cardboard. Obtain a sharp image of the circular hole on a screen `(MN)` using a second converging lens `(L_2)`, as shown in Fig. 11.11.
♦ Dissolve about `200 g` of sodium thiosulphate (hypo) in about `2 L` of clean water taken in the tank. Add about 1 to 2 mL of concentrated sulphuric acid to the water. What do you observe?
You will find fine microscopic sulphur particles precipitating in about `2` to `3` minutes. As the sulphur particles begin to form, you can observe the blue light from the three sides of the glass tank.
This is due to scattering of short wavelengths by minute colloidal sulphur particles. Observe the colour of the transmitted light from the fourth side of the glass tank facing the circular hole.
It is interesting to observe at first the orange red colour and then bright crimson red colour on the screen.
This activity demonstrates the scattering of light that helps you to understand the bluish colour of the sky and the reddish appearance of the Sun at the sunrise or the sunset.
Light from the Sun near the horizon passes through thicker layers of air and larger distance in the earth’s atmosphere before reaching our eyes (Fig. 11.12).
However, light from the Sun overhead would travel relatively shorter distance. At noon, the Sun appears white as only a little of the blue and violet colours are scattered. Near the horizon, most of the blue light and shorter wavelengths are scattered away by the particles.
Therefore, the light that reaches our eyes is of longer wavelengths. This gives rise to the reddish appearance of the Sun.