Diffraction is a fundamental wave phenomenon that occurs when a wave encounters an obstacle or passes through an opening and subsequently bends, spreads, or changes direction. Although people often associate diffraction with light, it is a universal behavior exhibited by all wave types, including sound waves, water waves, and matter waves such as electrons. Understanding diffraction requires viewing waves not as simple rays traveling in straight lines but as oscillating fields that interact with their surroundings in ways that reveal their wave nature.
When a wavefront meets an obstacle or slit, portions of the wave are blocked, while the edges can act as new sources of wavelets that emanate outward. This spreading effect becomes especially prominent when the size of the opening or obstacle is comparable to the wavelength of the wave. For instance, water waves passing through a narrow gap form circular wavefronts on the other side, illustrating visually how diffraction redirects energy into regions that would otherwise lie in shadow if waves behaved strictly like rays.
Light, though often treated as traveling in straight lines, exhibits diffraction whenever it encounters structures on the scale of its wavelength—roughly hundreds of nanometers for visible light. This behavior becomes apparent in everyday life through patterns such as the colored fringes around shadows or the shimmering effects seen on CDs and gratings. In a classic single-slit diffraction experiment, light passing through a narrow aperture creates a central bright fringe flanked by alternating light and dark bands. These fringes arise from the constructive and destructive interference of the secondary wavelets spreading from different parts of the slit. When waves reinforce each other, bright regions appear; when they cancel, darkness results.
The degree of diffraction depends strongly on the relationship between wavelength and aperture size. If the opening is much larger than the wavelength, the wave continues largely unaffected. As the opening narrows to near the wavelength’s size, diffraction becomes significant and the wave spreads widely. This is why low-frequency sound, which has a longer wavelength, can bend around corners more easily than high-frequency sound. It is also why radio waves of different wavelengths require differently sized antennas and propagate differently around obstacles.
Diffraction is not limited to single slits. When waves pass through multiple openings, such as in a double-slit experiment, an intricate interference pattern emerges from the combination of diffraction and superposition. This experiment famously underscores the quantum mechanical nature of particles: even individual electrons produce interference patterns over time, illustrating that matter itself carries wavelike properties. Similarly, diffraction gratings—structures with many closely spaced slits or grooves—create precise and predictable interference patterns that can disperse light into its component wavelengths with high resolution. This makes them indispensable in spectroscopy and optical instrumentation.
From a mathematical standpoint, diffraction can be modeled using the Huygens–Fresnel principle, which states that each point on a wavefront acts as a source of spherical wavelets. The superposition of these wavelets determines the wave’s subsequent shape. More advanced descriptions use Fourier analysis, treating apertures as spatial filters that transform wavefronts in predictable ways. This framework not only explains optical diffraction but also underpins modern imaging systems, telecommunications, and even quantum mechanics.
In practical terms, diffraction sets fundamental limits on the resolution of optical devices such as microscopes and telescopes. Because no lens can focus light to a point smaller than roughly one wavelength, all images produced by wave-based systems are blurred to some degree by diffraction. Improving resolution requires manipulating wavelengths, apertures, or exploiting techniques that circumvent classical limits, such as using shorter wavelengths (e.g., electrons in electron microscopes) or computational methods in modern imaging.
Diffraction is therefore far more than a curious behavior of waves—it is a cornerstone concept that connects classical wave theory, optical technology, and quantum physics. It shapes how we design scientific instruments, how we interpret the behavior of particles at the smallest scales, and how we understand the fundamental nature of waves themselves.