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) The milk sample was illuminated because milk is a colloidal solution and its particles are big enough to scatter the light, hence they scatter the light passing through it. The phenomenon observed is called "Tyndall effect".
(b) As salt solution is a true solution i.e., solute particle size is too small to scatter the light, hence, it does not show "Tyndall effect".
(c)Examples of colloid are gold sol, arsenius sulphide `(As_(2)S_(3))` sol., Blood etc.