ONE SLIT
One open slit still forbids directions — a·sinθ = mλ dark, the central hub twice a side band, every d/a-th double-slit order missing.
Use the simulation above to change the variables and play through the guided stages. The explanation below describes the default starting values; the simulation updates its explanation as you experiment.
Setup
One color of light, one narrow slit, a distant screen. A single opening — no second slit to interfere with — and yet the screen is not a plain bright band: a broad central glow flanked by dark gaps and faint side lobes. One slit already forbids whole directions.
Pair up
Treat the open slit as an endless row of tiny sources. Pair each with its partner half a slit away: when the light from the two slit edges differs by a whole wavelength, every pair cancels exactly. A wide-open slit manufacturing perfect darkness — diffraction, not two-slit interference.
Solve
The dark bands obey a·sinθ = mλ. On a planar screen the exact central width is 2L tan[asin(λ/a)]; 2Lλ/a is shown separately as the small-angle approximation, with its error quantified.
Build
The intensity is I₀·sinc²(πa·sinθ/λ): a bright central hub with rapidly fading side lobes. The drawing zooms the tiny slit to be visible and says so; the spacing on the screen is true. Slide the slit width and watch the hub breathe.
Audit
Audited: the sinc² is exactly zero at every a·sinθ = mλ to 1e-12, the central peak is exactly I₀, and a slit of width a set inside a double slit of separation d kills every d/a-th fringe — the missing orders, verified to the bit. One slit already carries the whole diffraction limit that bounds every eye and telescope.