With the help of an activity show the blue colour of the sky and the reddish appearance of the sun at the sunrise or sunset.
- Place a source of light S such as an electric bulb at the focus of a converging lens L1. This lens provides a parallel beam of light.
- Allow the light beam to pass through a transparent glass beaker B of capacity 500 mL.
- After passing through the beaker containing about 400 mL of water allow the light beam to pass through a circular hole made in cardboard C. Obtain a sharp image of the hole on a screen MN using another convex lens L2.
- Dissolve about 200 g of sodium thiosulphate (hypo) in water taken in the beaker B. Add about 1 to 2 mL of concentrated sulphuric acid with the help of an injection syringe dropwise in the beaker containing hypo solution.
- What do you observe?
You will notice fine microscopic sulphur particles precipitating in about 2 to 3 seconds. As the sulphur particles begin to form, you can observe the blue light from the sides of the beaker. This is due to scattering of short wavelengths by minute colloidal sulphur particles. Observe the colour of the light patch on the screen. It is interesting to observe the change in colour of the patch-from orange red colour in the beginning to bright crimson red colour later on the screen.
This activity demonstrates the scattering phenomena of light that helps us to understand the bluish colour of the sky in general and the reddish appearance of the sun at the sunrise or the sunset.