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 Fig. 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?

Answer:

(a) Milk is a colloidal substance. Particulate matter present in the milk makes the light scatter which results in Tyndall effect. Because of Tyndall effect the milk got illuminated.

(b) Salt is a homogenous solution. Small particles present in salt solution do not scatter light rays hence there will be no Tyndall effect. Since salt solution did not exhibit Tyndall effect light is not illuminated.

(c) Detergent solution and sulphur solution exhibit the Tyndall effect.

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