Aperture size, wavefront construction and diffraction

Aperture size, wavefront construction and diffraction

Diffraction can be built from Huygens' principle rather than memorised as a pattern. When a plane wave reaches a gap, every accessible point across the gap becomes a source of secondary wavelets. A wide gap contains many sources spread across a large opening, so the new envelope remains almost straight in the middle. A narrow gap behaves more like one source, producing strongly curved wavefronts. Wavelength matters because it sets the scale of the wavelets: a fixed gap appears narrower to a longer wavelength wave and diffraction increases.

Work it through

Imagine waves with 2 cm wavelength passing through a 2 cm gap: spreading is pronounced because wavelength and gap are comparable. If the same waves pass through a 20 cm gap, the wavefront is much less curved after the opening. This is why radio waves with long wavelengths can bend around obstacles more noticeably than visible light in many settings.

Mastery target

Construct a diffraction explanation from Huygens wavelets and compare gap width with wavelength to predict the shape of outgoing wavefronts.