Diffraction describes how waves spread out as they pass through an opening or bend around an obstacle. This phenomenon applies to all types of waves, including light, sound, and water waves. It shows that waves deviate from a straight path when encountering boundaries, explaining various observable phenomena in our environment.
How Waves Bend
The underlying mechanism of diffraction can be understood through the Huygens-Fresnel principle, which posits that every point on a propagating wavefront acts as a source of tiny, secondary wavelets. These wavelets then spread outward, and their collective sum forms the new wavefront as the wave continues to move forward. When a wave encounters an obstruction or a narrow opening, these secondary wavelets originating from the edges cause the wave to bend into the region that would otherwise be a shadow.
The extent to which a wave bends or spreads depends on the relationship between its wavelength and the size of the obstacle or opening it encounters. Diffraction effects are most pronounced when the wavelength is comparable to or larger than the dimensions of the object or aperture. Conversely, if the wavelength is much smaller than the obstacle, the wave will continue in a straighter path with minimal bending. This principle explains how different wavelengths interact with objects.
Everyday Instances of Diffraction
Diffraction is a common occurrence, observable in many daily situations. An example involves light passing through a very narrow gap, such as the space between two fingers held closely together. As the gap narrows, one might observe dark lines or a rainbow-like pattern, which results from light waves bending and interfering after passing through the small opening. Similarly, the iridescent, rainbow-like patterns seen on a compact disc (CD) or digital versatile disc (DVD) are caused by light diffracting off the microscopic grooves on the disc’s surface, which act like a diffraction grating.
Sound waves also demonstrate diffraction, explaining why we can hear someone speaking from around a corner or through an open doorway even if they are not directly visible. Sound waves bend around obstacles like walls or furniture, allowing the sound to reach our ears. This effect is particularly noticeable with low-frequency sounds, such as bass from music or a distant rumble of thunder, because their longer wavelengths enable them to diffract more effectively around larger obstacles compared to higher-frequency sounds.
Water waves provide another illustration of diffraction. When ocean waves approach a harbor entrance or pass around a breakwater, they do not simply stop at the barrier. Instead, the waves bend and spread into the sheltered area behind the obstruction, creating a calmer zone within the harbor. This allows wave energy to propagate into regions that would otherwise be shielded. Atmospheric phenomena, like crepuscular rays visible when sunlight streams through gaps in clouds, are also a result of light waves diffracting around water droplets or ice crystals.