The process in which the light rays are deflected by the particles of the medium through which they pass is called scattering of light.
When rays of light fall on the fine particles of a colloidal solution, the path of the beam is visible due to the scattering of light by the colloidal particles. This phenomenon is called ‘Tyndall effect’.
The scattering of light as it passes through a medium containing small particles is called ‘Tyndall effect’.
The colour of the scattered light depends on the size of the scattering particles. Very fine particles scatter shorter wavelengths (colours at violet end) by greater amount as compared to the light having longer wavelengths (colours at red end). If the size of particles is quite large, then, the scattered light is almost white.
Tyndall effect is observed when a fine beam of sunlight enters a room through a small hole and the light get scattered from the smoke or dust particles present in the room ; when sunlight passes through a canopy of a dense forest, tiny water droplets present in the air scatter the light.
The ‘danger’ signals are made of red colour because red colour is scattered least by the air particles, smoke or fog due to its longer wavelength. Hence, red colour can be seen in the same shade even at the large distance.
1. Place a strong source (S) of white light at the focus of a converging lens (L1). 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 (M) using a second converging lens (L2), 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.
2. 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.
3. 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. We conclude that very fine particles scatter mainly shorter wavelength light (blue light) and longer wavelength light (red light) gets transmitted.
Colour of the sun at sunrise and sunset
At the sunrise or sunset, the sun is near the horizon. Light reaching the eye of an observer travels a larger distance through the atmosphere. Thus, most of the blue light and shorter wavelength rays are scattered away by the air particles. Hence, the light that reaches the eye of an observer is of longer wavelengths (of red end). Thus, the sun appears reddish at sunrise or sunset.
Sun looks white at noon
At noon, the light has to travel a comparatively shorter distance through the atmosphere before reaching the eye of an observer. Thus, only a very little amount of blue or violet light is scattered away. Hence, the sun at noon looks almost white (see figure).
Clouds are generally white
The clouds are generally white because the larger particles like dust and water drops scatter light of all colours, almost equally and all the colours reach our eyes equally and combine to form white light.
(Session 2025 - 26)