The remote sensing experiment
Analysing light reflected from a substance can allow us to make inferences about the properties of that substance --- its composition or temperature for example. This is an example of remote sensing: a mechanism for determining information about an object without being able to manipulate it physically. In this experiment we will analyse the polarisation of laser light reflected from samples of both ice and liquid water.
Briefly speaking, the polarisation of an electromagnetic wave such as light refers to the pattern traced out by the wave's electric field. We will be mainly concerned with linearly polarised light --- in which the electric field is confined to one plane --- and unpolarised light --- which is a mixture of light of random polarisations, much like white light is a mixture of light of a range of colours.
In the experiment, we shine linearly polarised laser light on to a tray of water (or ice). The reflected light is then directed through a rotatable linear polariser (analysing polariser) on to a light-detecting device (a photodiode) which records the incident intensity. If the light passing through the linear polariser is already linearly polarised then the intensity of light detected will follow Malus' law as we rotate the analysing polariser [Above, (a)]; if, on the other hand, the light passing through is unpolarised then the intensity will not change as the polariser is rotated (b). If the incident light is partially linearly polarised and partially unpolarised, then the intensity curve is intermediate between these extremes (c). A measure of the relative intensity of the polarised and unpolarised components, or the degree of depolarisation, is the specularity, S:
When measuring the specularity in our experiment, we are measuring how much of the light reflected from the surface of the water (or ice) retains the original linear polarisation, and how much gets randomised, or depolarised. For light reflecting from liquid water, we expect that the light will reflect almost entirely from very close to the surface and completely retain its original polarisation (this is termed specular reflection, and S=1). However, when the beam is incident on a sample of ice some of the light penetrates into the sample and is scattered many times by the individual crystals inside before leaving the surface again. Because of this scattering and a property of ice crystals known as birefringence, whereby the plane of polarisation of light passing through the crystal is rotated, the light that is scattered from the volume of the ice gets depolarised (specularity S<1). The aim of the experiment is to try and observe this depolarising effect of ice by directly measuring the specularity of reflection. We will also attempt to measure the specularity continuously through the ice/water phase transition, as is necessary in some remote sensing applications (in ice detectors on aircraft, for example).