Diffraction describes the bending or spreading of waves as they encounter an obstacle or pass through an opening. This phenomenon causes waves to deviate from their straight-line path. First observed and named by Italian scientist Francesco Maria Grimaldi in 1660, diffraction is a fundamental characteristic of all wave types, including light, sound, water, and other forms of electromagnetic radiation.
The Fundamental Condition for Diffraction
Diffraction occurs most noticeably when a wave’s wavelength is similar in size to, or larger than, the obstacle or opening it encounters. If the object or aperture is significantly larger than the wavelength, the bending effect is minimal and often goes unnoticed, making the wave appear to travel in a straight line.
This wave behavior can be understood through Huygens’ principle, which proposes that every point on a wavefront acts as a source of new, secondary wavelets. These wavelets spread out from their source at the same speed as the original wave. When a wavefront reaches an edge or an opening, the wavelets generated at that boundary spread into the area that would otherwise be a shadow.
The combined effect of these spreading wavelets creates the observed diffraction pattern, which often features areas where wave energy is concentrated or diminished. For example, low-frequency sound waves have long wavelengths, allowing them to easily bend around large objects like buildings or corners. In contrast, visible light waves have very short wavelengths, which is why light does not typically bend around everyday objects, and we see sharp shadows.
Common Observations of Diffraction
The principles of diffraction are evident in many everyday phenomena, particularly involving light. When light passes through a very narrow slit, it does not simply project a sharp image of the slit. Instead, the light spreads out, forming a distinct pattern of alternating bright and dark bands on a screen. This spreading is most apparent when the slit’s width is comparable to the light’s wavelength.
The shimmering, rainbow-like colors visible on the surface of a Compact Disc (CD) or Digital Versatile Disc (DVD) are another common example of diffraction. These discs contain microscopic spiral grooves that store digital information. On a CD, these grooves are spaced approximately 1.6 micrometers apart, while on a DVD, they are about 0.74 micrometers. These closely spaced grooves act as a reflective diffraction grating, splitting white light into its constituent colors as it reflects.
Diffraction also contributes to atmospheric optical effects. Light can diffract around tiny water droplets in clouds, producing subtle pastel shades of blue, pink, purple, and green, sometimes observed as a “silver lining.” Phenomena such as crepuscular rays, where beams of sunlight appear to radiate through gaps in clouds, also involve light bending due to diffraction.
Diffraction Across the Electromagnetic Spectrum
Diffraction applies universally to all types of electromagnetic radiation, spanning a vast range of wavelengths from X-rays to radio waves. The fundamental condition for observable diffraction—the comparison between wavelength and obstacle size—remains consistent across this entire spectrum.
X-ray diffraction (XRD) is a scientific technique that relies on the extremely short wavelengths of X-rays, typically ranging from 0.1 to 10 nanometers. These wavelengths are comparable to the spacing between atoms in crystalline materials. When X-rays interact with the regularly arranged atoms within a crystal, they diffract in specific directions. Analyzing the resulting diffraction pattern allows scientists to determine the precise atomic and molecular structure of the crystal.
Conversely, radio waves have very long wavelengths, often ranging from centimeters to kilometers. Because of these long wavelengths, radio waves readily diffract around large obstacles such as hills, buildings, and even the curvature of the Earth. This property is why radio signals can be received even when a direct line of sight to the transmitting antenna is obstructed.