C.3.11—Diffraction around bodies and through apertures

Diffraction is wave spreading

A wave diffracts when its wavefront bends into the region behind an obstacle or spreads after passing through an aperture. The wave remains in the same medium, so diffraction itself does not require a change of speed or frequency.

Compare wavelength with the opening or body

Spreading is most noticeable when the aperture width or obstacle size is comparable to the wavelength. An aperture much wider than the wavelength gives a broad central region that travels nearly straight with limited edge spreading; narrowing the aperture increases the angular spread.

Wavefront–ray representation

Before a straight aperture, incident wavefronts are parallel. Beyond a narrow aperture, draw curved outgoing wavefronts; rays stay perpendicular to them and fan outward. Around a body, wavefronts curve into the geometrical shadow.

Do not confuse mechanisms

Refraction is a direction change caused by a speed change at a boundary. Diffraction is spreading caused by an edge or aperture, even when the medium is unchanged.