i. Doppler effect of light is used to determine the radial velocities of distant galaxies.
ii. It is used to measure the speed of rotation of the sun.
a. The east and west edges of the sun are photographed. Each contains absorption lines due to elements such as iron vaporised in the sun and also some absorption lines due to oxygen in the earth’s atmosphere.
b. When the two photographs are put together so that the oxygen lines coincide, the iron lines in the two photographs are displaced relative to each other.
c. In one case, the edge of the sun approaches the earth and in the other, the opposite edge recedes from the earth. Measurements show a rotational speed of nearly 2 km/s.
