Diffraction is a physical phenomenon that occurs when light encounters an obstacle or passes through a small aperture (narrow slit), causing it to deviate from its straight-line path and bend around the obstacle to continue propagating. The resulting pattern of light and dark bands or rings is called a diffraction pattern.
For diffraction to occur, certain conditions must be met: the size of the obstacle or aperture must be comparable to or smaller than the wavelength of the light. Since the wavelength of light is very short, usually only a fraction of a micrometer, diffraction is most clearly observed when light interacts with small obstacles like pinholes, narrow slits, or thin wires.
The mechanism behind diffraction is the result of the superposition of coherent light waves. When light from a source shines on a small aperture or obstacle, the edges of the aperture or obstacle can be considered as point sources of light (according to Huygens’ Principle). These point sources are coherent, and the light they emit interferes with each other, forming alternating light and dark bands on a screen. The following types of diffraction patterns can be produced:
● Single-slit diffraction: Creates alternating light and dark bands with varying spacing.
● Circular aperture diffraction: Produces concentric light and dark rings with varying spacing.
● Disc diffraction: Similar to circular aperture diffraction, producing concentric rings, with a bright spot in the center called the “Poisson spot.”
Depending on the relative distances between the light source, the diffraction screen, and the receiving screen, diffraction can be classified into two types:
● Fresnel diffraction: Occurs when the distance between the light source and the diffraction screen, or the distance between the receiving screen and the diffraction screen, is finite. In this case, the incident light or the diffracted light is not parallel.
● Fraunhofer diffraction: Occurs when both the distance between the light source and the diffraction screen, and the distance between the receiving screen and the diffraction screen, are infinite. Here, both the incident light and the light reaching any point on the receiving screen are parallel. In laboratory settings, Fraunhofer diffraction conditions can be achieved using lenses.
Applications of Diffraction in Analysis Techniques:
● Spectral analysis: Diffraction grating spectrometers use the diffraction of light to analyze different wavelengths.
● Structural analysis: Diffraction patterns can “magnify” fine structures, allowing scientists to analyze structures, as seen in X-ray crystallography.
● Diffraction imaging: In coherent light imaging systems, the concept of double diffraction is introduced, which led to advancements in spatial filtering and optical information processing.
● Holography: The reproduction of the wavefront through diffraction is a crucial step in holography.
● Crystal structure determination: X-ray diffraction is a key method for determining the structure of crystals.
Everyday Examples of Light Diffraction:
● Sunlight through tree leaves: When sunlight passes through the gaps between leaves, it creates a pattern of alternating light and dark spots on the ground.
● Rainbow: Although rainbows mainly involve refraction and reflection, diffraction at the edges of raindrops also plays a role.
● Colored edges when blocking sunlight with fingers: When blocking sunlight with your fingers, colored fringes can be seen along the edges of your fingers.
● Colorful stripes on a CD: The microscopic grooves on a CD cause diffraction, producing colorful patterns.
● Diffraction from gratings: When light shines on a periodic structure like a grating, diffraction occurs, forming alternating light and dark bands.
Post time: Oct-17-2024