The wavefronts of light coming from a distant source of unknown shape are nearly

Which of the following sources gives best monochromatic light?

The wave fronts of a light wave travelling in vacuum are given by x + y + z = c. The angle made by the direction of propagation of light with the X-axis is

The inverse square law of intensity (i.e., the intensity ∝ 1/r² is valid for a

Two sources are called coherent if they produce waves