Spreading through openings
Waves spread when they pass openings or encounter obstacles, departing from a simple ray picture. Sound, water waves and light all diffract. The effect becomes especially noticeable when an opening is comparable to the wavelength, explaining why everyday sound bends around doorways more visibly than light.
References: [1]
The single-slit pattern
Monochromatic light passing through a narrow slit produces a broad central maximum and weaker side maxima. Contributions from different positions across the slit combine with different path lengths. Changing slit width changes the angular pattern, so the screen does not simply show an enlarged geometric image.
References: [1]
From patterns to spectra
A diffraction grating combines contributions from many regularly spaced lines or openings. Different wavelengths form maxima at different angles, allowing mixed light to be separated for spectroscopy. Interpreting a spectrum still requires the grating geometry, diffraction order and calibration of the instrument.
References: [2]
Resolution and structural measurement
A circular aperture spreads a point source into a finite diffraction pattern, limiting separation of nearby optical targets. Matter waves also diffract: electron patterns from crystals support de Broglie’s description. Such measurements reveal structure through comparison with models, rather than by directly photographing every component.