Young’s Double Slit Experiment

 

Thomas Young (1773-1829) performed the first visible-light interference experiments using a clever technique to obtain two coherent light sources from a single source. When a single narrow slit is illuminated, the light wave that passes through the slit diffracts or spreads out. The single slit acts as a single coherent source to illuminate two other slits. These two other slits then act as sources of coherent light for interference.

 

 Young technique for illuminating two slits with coherent light. The single slit on the left serves as a source of coherent light.

 

In Young's interference experiment, incident monochromatic light is diffracted by slit a, which then acts as a point source of light that emits semicircular wavefronts. As that light reaches screen S₂. it is diffracted by slits b and.c, which then act as two point sources of light. The light waves traveling from slits b and c overlap and undergo interference, forming an interference pattern of maxima and minima on viewing screen F. This figure is a cross section; the screens, slits, and interference pattern extend into and out of the page. Between screens s₂ and F, the semicircular wavefront's centered on c depict the waves that would be there if only c were open. Similarly, those centered on c depict wave that would be if only c were open. Points of interference maxima from visible bright rows called bright bands, bright fringes, or (loosely speaking) maxima that extend across the screen