Definition and Core Principle
Fraunhofer diffraction, also known as far-field diffraction, describes the diffraction pattern observed when both the light source and the observation screen are effectively at infinite distances from the diffracting obstacle or aperture. Under these conditions, the incident light waves and the diffracted waves can be treated as planar, allowing for a simplified mathematical analysis compared to near-field (Fresnel) diffraction.
Key Characteristics and Conditions
The defining characteristic of Fraunhofer diffraction is the parallelism of the incident and diffracted light rays. In practical settings, these 'infinite' distances are achieved by using converging lenses. A collimating lens is placed between the light source and the aperture to make the incident light parallel, and another focusing lens is placed after the aperture to bring the parallel diffracted rays to a focus on the observation screen.
Mathematical Representation and Examples
Mathematically, Fraunhofer diffraction patterns are typically derived using Fourier transforms, which describe how the spatial distribution of light is transformed by the aperture. Common examples include the diffraction pattern produced by a single slit, a double slit, or a circular aperture. The pattern from a circular aperture is known as the Airy disk, characterized by a central bright spot surrounded by concentric dark and bright rings.
Importance and Applications
Fraunhofer diffraction is fundamental to understanding the wave nature of light and has significant applications across various scientific and engineering disciplines. It is crucial for designing and evaluating optical instruments, as it defines the theoretical limit of their resolving power. Fields such as astronomy (telescope resolution), microscopy (imaging capabilities), and spectroscopy rely heavily on the principles of Fraunhofer diffraction.