Single-Slit Diffraction

A single slit is not a point — every part of its opening re-radiates as a Huygens wavelet, and those wavelets interfere to produce a bright central band flanked by weaker side maxima. The counter-intuitive result: the narrower the slit, the wider the light spreads. Students vary the slit width a, screen distance D and wavelength λ, and can watch the wavelets superpose (Wavefronts) or the steady pattern build up (Intensity). Central band width w = 2λD/a.