Why Waves Bend Around Corners

You hear someone talking around a corner but you never see them until you step into view. Same hallway, same corner, totally different behavior, because sound and light are both waves but with wildly different wavelengths. Sound wavelengths run from centimeters to meters, close to the size of doorways and corners, so sound diffracts strongly and curls around the edge. Visible light has a wavelength around 500 nanometers, millions of times smaller than any ordinary opening, so its diffraction is there but far too slight to notice. That is the whole rule: diffraction matters when wavelength and opening size are comparable.

Huygens' principle explains the mechanism. Treat every point on a wavefront as if it were a brand new source sending out its own tiny circular wavelet. Add up all those wavelets and you get the next wavefront. When a wave hits a barrier with a gap, the wavelets near the edges of the gap are not cancelled out by neighboring wavelets on one side, so they spread into the shadow region behind the barrier. A wide-open wavefront has wavelets canceling in every direction except straight ahead, which is why waves look like they travel in straight lines through large openings.

This sets up the rest of the unit. Once you accept that waves spread through gaps, you can ask what happens when two spreading wavefronts overlap, which is interference, and what happens when many narrow slits diffract together, which builds the sharp bright and dark fringes of diffraction gratings and single and double slit patterns.