A group of students took an old shoe box and covered it with a black paper from all sides. They fixed a source of light (a torch) at one end of the box by making a hole in it and made another hole on the other side to view the light. They placed a milk sample contained in a beaker/tumbler in the box as shown in the Figure. They were amazed to see that milk taken in the tumbler was illuminated. They tried the same activity by taking a salt solution but found that light simply passed through it ?

(a) Explain why the milk sample was illuminated? Name the phenomenon involved.
(b) Same results were not observed with a salt solution. Explain.
(c) Can you suggest two more solutions which would show the same effect as shown by the milk solution?

Explanation

(a) Milk sample is a colloidal sol also called emulsion. It got illuminated because of the scattering of light on passing through it. The phenomenon is known as Tyndall effect.
(b) The salt solution is a true solution and not a colloidal solution. It will not show any Tyndall effect.
(c ) A solution of sulphur in water and a solution of starch in water.