The structure and properties of ice
The water molecule
Water, chemical formula H2O, is a molecule particularly abundant on the surface and in the atmosphere of earth in all its phases — vapour, water, and ice. Many of ice's properties ultimately derive from two very simple facts about the structure of a single water molecule: fistly, it contains hydrogen bonded to a highly electronegative element (oxygen), and secondly its ground state configuration is bent, as shown in the following diagram (1Å = 10-10m):
The first of these two properties allows water molecules to hydrogen bond: the electron from the hydrogen atom is so strongly attracted to the covalently bonded oxygen that the hydrogen nucleus (just one proton) acts as a positively charged 'hook' that can grapple on to the negatively charged oxygen center of another water molecule. The binding between the proton and the oxygen in the next molecule is not as strong as the covalent bonds within one molecule, but it is strong enough to make the boiling point of water much higher than the heavier H2S molecule, for example.
Ice crystals
In ice each oxygen molecule is covalently bonded to two hydrogens and can accept two more hydrogen bonds. This leads to each oxygen atom being bonded to four others, forming a tetrahedral structure. However, the positions of the covalently bonded hydrogens — that is, which of the four bonds surrounding a given oxygen are covelent and which are hydrogen bonds — is not fixed (unless the ice is produced under specific laboratory conditions). Within one ice crystal any distribution can occur which follows the two so-called ice rules: (1) each oxygen has two adjacent hydrogen atoms, and (2) there is only one hydrogen atom per oxygen-oxygen bond. This freedom is illustrated in the figure below, which shows a single layer of ice crystal (short dangling bonds connect to oxygen atoms in the layers above and below) with one possible arrangement of the hydrogen atoms.
Optical properties of ice crystals and scattering of light from polycrystalline ice
One of the most interesting optical properties of ice crystals is their birefringence: light rays travelling in different directions through the crystal structure can see different refractive indices.
A calcite crystal showing double refraction due to birefringence: components of a light ray travelling parallel and perpendicular to the optical axis travel at different speeds, leading to two refracted images. Image: wikimedia.org
Birefringence means that light which enters the crystal neither perpendicular nor parallel to the extraordinary optical axis will emerge with its polarisation rotated: how much it is rotated by depends on the thickness of the crystal. However, ice made by cooling water in air at atmospheric pressure without any special procedures will be polycrystalline (composed of many, randomly oriented, individual ice crystals). Rather than having its polarisation rotated, a beam of light passing through a sufficiently thick block of polycrystalline ice will end up depolarised, as the rotations caused by each of the randomly oriented individual crystals in the block will cancel each other out.
This effect is also important when reflecting light off the ice, as in our experiment. The surface of a block of polycrystalline ice is not homogeneous, and a beam of light incident on it will partially penetrate into the volume of the block. This penetrating light will reflect off and pass through many individual crystals, getting depolarised as it does so, and some of it will eventually leave the material in the same direction as the surface reflection. This means the reflected beam will now be partially depolarised, and this is the effect we aim to demonstrate experimentally.