CBSE Class 10 Science 2017 · Paper 531/1 · Question 16

Activity: Place a strong source of white light at the focus of a convex lens so that a parallel beam passes through a transparent glass tank of clear water, then through a circular hole in a cardboard; a second convex lens forms a sharp image of the hole on a screen. Dissolve about 200 g of sodium thiosulphate in about 2 L of water in the tank and add about 1 to 2 mL of concentrated sulphuric acid. Observation: in 2 to 3 minutes fine particles of sulphur begin to form. Blue light is then seen from the three sides of the tank, while the image of the hole on the screen (the transmitted light, seen through the fourth side) appears at first orange-red and then bright crimson red. Explanation: the very fine colloidal sulphur particles scatter the short wavelengths (blue) much more than the long ones (red). The scattered blue light is seen from the sides, and the transmitted beam, having lost much of its blue, looks red. This demonstrates the scattering of light, which also explains the blue colour of the sky and the reddish Sun at sunrise and sunset.