Diffraction by Opaque and Transparent Particles

Abstract

Diffraction by opaque and transparent particles which present a circular cross-section to a collimated quasi-monochromatic beam of light is discussed theoretically and experimentally. The plane in which the diffraction is recorded is at a sufficient distance from the plane containing the particles for the ap-proximation of Fraunhofer diffraction to be made--but not in the focal plane of a lens. The intensity distribution in the Fraunhofer pattern consists of several terms but the dominating one involves the Airy pattern of a circular aperture multiplied by a sine function, which is independent of the size of the particles. This term, which represents the interference between the light diffracted by the particle and the coherent background, may be removed by spatially filtering the diffraction pattern formed in the focal plane of a lens. A second lens re-transforms the remaining pattern to produce an image of the original particles. The film planes are set at known distances from the image plane so that Fraunhofer diffraction patterns of the individual particles are recorded. Using a pulsed ruby laser as a source, diffraction patterns of moving particles have been photographed. The results obtained indicate that this technique should prove quite useful in the measurement of small particles, aerosols, fog droplets, etc.