An experimental analogs study of the light scattering characteristics of ice-like hexagonal crystals.

摘要

An experimental apparatus to measure the scattering properties of hexagonal ice-like crystals at the helium neon laser wavelength of 632.8 nanometers has been designed and built. This method allows direct comparison to theoretical calculations because we know the optical properties, the spatial orientation and the size and shape of the crystal being studied.; The instrument consists of an array of 36 highly sensitive and linear photo diode detectors that are positioned to measure the light between the scattering angles of 2.8{dollar}spcirc{dollar} and 177.2{dollar}spcirc{dollar} in approximately 2.5{dollar}spcirc{dollar} increments and at any desired azimuthal angle. A system of electro-mechanical positioners is used to orient and place the scattering center in the incident laser beam. This detector array is calibrated such that the experimental results are corrected to the actual light scattered by the sample to within approximately 6% at all applicable scattering and azimuthal angles. The effective operation and calibration of the experimental apparatus were tested with a glass sphere and a glass fiber configured as an infinite cylinder.; Sodium fluoride (NaF), which has an index of refraction = 1.336 (very similar to that of ice), has been formed into several different crystals shapes to simulate ice crystals seen in nature, i.e., columns, plates and an aggregate. The experimentally measured scattering pattern and phase function of a NaF hexagon oriented in a manner that confines its scattering pattern to a single plane agrees with theoretically derived results based on ray tracing methods. Because the NaF model is not geometrically perfect, and because its surfaces are not interferometrically smooth, there are slight differences between the experimental and theoretical results.; The light scattered from a NaF hexagonal crystal oriented to mimic the naturally occurring crystal that produces Parry arcs, a NaF plate with a rough surface, and a NaF aggregate has been measured with this apparatus in three dimensions, and the measured phase functions compare favorably to theoretical expectations calculated using ray tracing methods.