What is the shape of the wavefront in each of the following cases:(a) Light diverging from a point source.
(b) Light emerging out of a convex lens when a point source is placed at its focus.
(c) The portion of the wavefront of light from a distant star intercepted by the Earth.
The observed wavelength of light coming from a distant galaxy is found to be increased by 0.5 % as compared with that coming from a terrestrial source. The galaxy is