14.4 Single-Slit Diffraction (AHL)

Treat each point across slit width b as a secondary source. At observation angles where paired points in the aperture emit waves that arrive out of phase, cancellation occurs. Pairing the top half and bottom half gives the first minimum; subdividing aperture regions gives higher minima.

The central maximum is brightest and roughly twice the angular width of side maxima. Its width increases when wavelength increases or slit width decreases. This is the key aperture-resolution tradeoff: narrowing the aperture improves geometric selectivity but broadens diffraction spread.

These approximations are powerful for trend questions. If b halves while lambda and D remain fixed, central maximum width doubles. If lambda increases, minima move farther apart. Keep this proportional reasoning in mind before calculations so your numeric answer can be sanity-checked.

Real two-slit systems have finite slit width, so each slit diffracts. The interference fringes then sit inside a broader single-slit envelope. Near the center, fringes are high intensity; farther out, fringe intensity drops and some expected interference orders can be missing where envelope minima occur.

Interpreting this combined pattern correctly is a common AHL discriminator. If the question asks why outer fringes fade, do not answer with source brightness reduction. The correct explanation is envelope suppression from finite slit width.