Why does light passing through a single very narrow slit spread into a broad bright band instead of a sharp line?
That's single-slit diffraction: every point of the slit acts as a wavelet source, and these wavelets interfere.
That's single-slit diffraction: every point of the slit acts as a wavelet source, and these wavelets interfere. They reinforce straight ahead (the wide central maximum) and cancel at angles where the path difference across the slit is a whole wavelength, giving dim fringes on either side. The narrower the slit (comparable to λ), the wider the central band spreads.
Key point
Single-slit diffraction: central maximum widens as the slit narrows toward λ.
Common mistake
expecting a narrow slit to give a narrow, sharp line of light.
Memory tip
narrow the slit → wider the central diffraction band.
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