These effects also occur when a light wave travels through a medium with a varying refractive index, or when a sound wave travels through a medium with varying acoustic impedance – all waves diffract, including gravitational waves, water waves, and other electromagnetic waves such as X-rays and radio waves. If there are multiple, closely spaced openings (e.g., a diffraction grating), a complex pattern of varying intensity can result. This is due to the addition, or interference, of different points on the wavefront (or, equivalently, each wavelet) that travel by paths of different lengths to the registering surface. The characteristic bending pattern is most pronounced when a wave from a coherent source (such as a laser) encounters a slit/aperture that is comparable in size to its wavelength, as shown in the inserted image. In classical physics, the diffraction phenomenon is described by the Huygens–Fresnel principle that treats each point in a propagating wavefront as a collection of individual spherical wavelets. Infinitely many points (three shown) along length d on the registering plate Italian scientist Francesco Maria Grimaldi coined the word diffraction and was the first to record accurate observations of the phenomenon in 1660. The diffracting object or aperture effectively becomes a secondary source of the propagating wave. The waves interfere with each other so that there is constructive interference in some areas (left-hand picture) and destructive interference in other areas (right-hand picture).Not to be confused with refraction, the change in direction of a wave passing from one medium to another.Ī diffraction pattern of a red laser beam projected onto a plate after passing through a small circular aperture in another plateĭiffraction is the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. In the image below, two sources – labelled Sound 1 and 2 – are aligned one above the other. When the same pitch or frequency sound wave is produced from two sources, a pattern of interference is produced. Sound waves and pitchīecause sound travels outwards from a central source, waves interact in interesting patterns. A sound wave with the beat pattern in diagram D will have a volume that varies at a regular rate – you can hear a pulse or flutter in the sound. The resulting wave has points of constructive interference and destructive interference. When we hear the sound of two different musical notes, as shown in diagram C, we hear a complex waveform we think of as harmony.ĭiagram D shows beats – when two sound waves are nearly the same frequency but slightly different. The result of any combination of sound waves is simply the addition of the various waves. They detect the sounds coming into the ear and produce sounds with equal volume but with the peaks and troughs reversed, resulting in near silence. Noise-cancelling headphones work on this principle. The result is a cancellation of the waves. The result is a wave that has twice the amplitude of the original waves so the sound wave will be twice as loud.ĭestructive interference is when similar waves line up peak to trough as in diagram B. With constructive interference, two waves with the same frequency and amplitude line up – the peaks line up with peaks and troughs with troughs as in diagram A above.
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