ABSTRACT
A source of light consists of a large number of atoms which emit wave-trains. For simplicity, it is assumed that all the atoms emit wave-trains of the same duration, called the coherence-time. The monochromaticity of a source is related to the coherence time. The smaller the vapour pressure of mercury in the lamp, the smaller is the band-width of the radiation emitted and the longer is the coherence time. The Michelson interferometer is illuminated in monochromatic light and is adjusted for approximately zero path difference. The theorem of Van Cittert-Zemike enables us to visualize a decrease in the contrast of fringes when the source is enlarged. On reappearance the fringes have low contrast and the bright and dark fringes are observed to have interchanged their position.
