White or polychromatic light sources are vastly abundant in nature and lie in our most basic
understanding of the theory of light, beginning from stars like our Sun and extending to every
common household light bulb or street lamp. In this paper, I present concepts of white light
interferometery using a common-path Sagnac interferometer, manifested in a straightforward
laboratory experiment. I further show the use of this as a Fourier transform spectrometer while
presenting a basic overview of the theoretical concepts and spectrum of different light sources
obtained experimentally. This work, both experimentally and analytically, is suitable for
upper-level undergraduate physics or engineering courses where electromagnetic theory and optics are
discussed. The experiment and theory presents important deep concepts and aspects in modern optics
and physics that every science student should acquire.