Single-Slit Diffraction

Picture the slit not as one gap but as a row of tiny point sources, each sending out its own wavelet, because every point on the wavefront that reaches the opening re-emits light. Straight ahead, all these wavelets travel almost the same distance to the screen, so they arrive in step and add up into a bright central band. Off to the sides, the paths from different parts of the slit differ in length. At certain angles those path differences cause every wavelet from the top half of the slit to cancel exactly with one from the bottom half, pairwise, across the whole opening. That total cancellation is a dark fringe. The condition for the first one is sin(theta) = lambda/a, where a is the slit width and lambda the wavelength.

Notice what that equation says about slit width: theta and a sit on opposite sides of the relationship, so a smaller a forces a larger theta. Shrink the slit and the dark fringes move farther from center, which drags the whole pattern wider. That feels backwards if you're used to thinking a narrower opening gives a sharper, thinner image, like squinting to see detail. But diffraction isn't imaging — it's interference across the slit, and a narrower slit gives waves less room to build up that pairwise cancellation close to the center, so they need a bigger angle to do it.

This sits right next to double-slit interference in the unit: double-slit fringes come from two separated sources interfering, while single-slit diffraction comes from infinitely many sources within one opening interfering with each other. In real double-slit setups both effects overlap, with the single-slit pattern acting as an envelope that shapes the brightness of the double-slit fringes.