Light Bends Around Corners
When light passes through a narrow slit, it doesn't simply project a sharp shadow of the slit onto a distant screen. Instead, it spreads out, creating a broad central bright band flanked by alternating dark and bright fringes. This is diffraction — direct evidence that light behaves as a wave. The effect becomes dramatic when the slit width approaches the wavelength of light. If slits were infinitely narrow, light would spread in all directions equally. The finite slit width creates the characteristic sinc-squared intensity pattern.
The Single-Slit Pattern
The intensity distribution from a single slit follows I(θ) = I₀(sin β/β)², where β = πa sin(θ)/λ. The central maximum is twice as wide as each side lobe and contains about 84% of the total transmitted light energy. Dark fringes (minima) occur where a sin(θ) = mλ for integer m, corresponding to positions where light from different parts of the slit destructively interferes. The narrower the slit, the broader the pattern — a direct manifestation of Heisenberg's uncertainty principle applied to photon position and momentum.
Resolution and the Diffraction Limit
Diffraction sets a fundamental limit on the resolution of all optical instruments. A circular aperture of diameter D cannot resolve two point sources closer than θ = 1.22λ/D — the Rayleigh criterion. This is why radio telescopes (long wavelengths) must be enormous to achieve useful resolution, while electron microscopes (extremely short de Broglie wavelength) can resolve individual atoms. The diffraction limit is not a technology problem to be solved but a fundamental consequence of wave physics.
Beyond the Single Slit
Single-slit diffraction is the building block for understanding more complex optical phenomena. Double-slit interference (Young's experiment) combines diffraction with two-source interference to produce the iconic fringe pattern. Diffraction gratings — arrays of thousands of slits — produce extremely sharp spectral lines used to analyze starlight and characterize materials. Even the Fourier transform has a direct physical analog in Fraunhofer diffraction: the far-field diffraction pattern is the Fourier transform of the aperture function.