ABSTRACT
The concept of first-order coherence was introduced in classical theory of light much earlier than that of the quantum theory. The degree of first-order coherence is defined to quantify the interference between temporally delayed or spatially separated electromagnetic waves. The superposed radiations are defined as first-order coherent if the interference fringes exhibit 100% modulation, or first-order incoherent if no interference fringes are observable. The radiation fields are considered as partial coherent if the modulation is less than 100%, however, greater than zero. The higher the degree of first-order coherence, the higher interference visibility people could observe. The concern leads to the concepts of spatial coherence and the degree of first-order spatial coherence. The spatial coherence of light is directly related to the transverse dimension, or the angular size of the light source. Taking up an early suggestion by Fizeau, Michelson designed a stellar interferometer based on the mechanism of Young’s double-pinhole interference.
