A beam of light is allowed to pass through two beakers A and B, containg a true solutionand a colloidal solution respectively. What do you observe ? Name the phenomenon responsible for your observation.

Explanation

Aim of the activity: To study Tyndall effect or scattering of light by colloidal particles.

(1) Place a strong source of white light (S), a convex lens `[L_(1)]`, a transparent glass tank [T] containing clear water [about 2 litres], a cardboard [C ] with a circular hole, another convex lens `[L_(2)]` and a screen [MN] in a straight line as shown in the figure.
(2) Focus white light from S into `L_(1)` so that a parallel beam of light is obtained.
(3) This beam of light passes through T, the hole in the cardboard, `L_(1)` and finally falls on the screen.
(4) Dissolve about 200 g of sodium thiosulphate (hypo-chemical formula `Na_(2)S_(2)O_(3)`) in the tank water and later add 1-2 ml of concentrated sulphuric acid into it.
Inference : Sulphur particles starts precipitating in about 2-3 minutes and blue light starts coming from three sides of the glass tank. This is because minute sulphur particles scatter light of short wavelengths [blue colour] to a maximum extent. This helps us to understand why or sky appears blue.
We also observe that initially orange light comes out of the fourth side of the tank [the one facing the circular hole] and falls on the screen, Later the colour changes by scattering and the least scattered red light comes out of the colloidal solution. This helps us to understand why the sun appears red during sunrise or sunset.