There were two competing theories about light: the wave theory and the particle theory.Newton considered light was composed of tiny particles (corpuscular theory) and Huygens explained reflection and refraction through the wave theory.Newton's theory remained unchallenged for about 100 years until Thomas Young demonstrated interference by the wave theory.Young's double slit apparatus consists of two illuminated slits which act as coherent sources of waves with constant phase difference that spread out by diffraction and overlap resulting in interference.Alternate bright and dark fringes known as Young's fringes are seen on the screen.