Fluctuations in Scattered Light

Fluctuations in scattered light constitute some of the most familiar natural optical phenomena, including the twinkling of starlight, the glittering of sunlight on a rippled water surface and the shimmering of distant objects on a hot day [1]. These effects are geometrical in origin being a consequence of refraction by large-scale random variations in refractive index. Fluctuations of this kind may be caused by density variations within a propagation medium, such as the atmosphere, or by roughness of the interface between media having differing dielectric constants. Although significant dispersion may also be observed (as for example in the case of the colourful twinkling of a star low down in the sky) for the most part they are white light optical effects that are independent of wavelength within the visible spectrum. With the advent of the laser a new range of coherent light scattering phenomena became visible to the naked eye. In particular, the random interference effect that has come to be known as laser speckle commonly occurs in laboratory laser light scattering experiments [2]. Indeed laser light scattering generates many diffraction, interference and geometrical optics effects that can be observed with the naked eye. It is worth emphasising, however, that a similar range of effects is generated at other frequencies of the electromagnetic spectrum. One very familiar phenomenon of this kind is the fading of short wave radio reception due to ionospheric fluctuations. Many early theoretical results were derived as a consequence of the detection of fluctuations at these longer wavelengths. For example, an early theoretical description of random interference effects was developed to explain the fluctuating radar return from raindropstwenty years before the visual appearance of the equivalent lasergenerated phenomenon gave rise to the term “speckle”.