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  • 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.

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. 2.4. 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?

Answers (1)

Scattering of light through heterogeneous mixture is called Tyndall effect.
Examples of Tyndall effect:
(i) In Fog, Beam of head light of car is visible.
(ii) Sunlight passing through dust particle will be visible.
(iii) Path of light rays seen in front of the projector in a cinema hall.
The Tyndall effect is shown by mixture, where mixture has tendency to scatter the light of shorter wavelength and transmit the light of larger wavelength.
The Tyndall effect is shown by heterogeneous mixture such as colloid.
(a) Milk is a colloidal solution hence shows Tyndall effect.
(b) Salt solution is homogeneous therefore do not show Tyndall effect and  they do not scatter light.
(c) Detergent mixture and air with dust will show Tyndall effect.

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