I.1 Properties of
light
Visible light is that part of the electromagnetic radiation for which the human eye is sensitive, which means the range between approximately 700 and 420 nm. Within this range every wavelength has its own colour (Fig. 1.1). Light with one particular wavelength is called monochromatic.
Fig.1.1 The electromagnetic spectrum
The speed of light is maximal in vacuum, i.e. c = 300,000 km/sec. When a space is filled with material, the speed will decrease. This is reflected in the relation c/vm = nm, where vm and nm are the speed of light in the medium and the refractive index of that medium. nm is always larger than 1.
We can consider light as a transverse vibrating wave (Young and Fresnel theory). In transverse vibrations the vibrational direction is not restricted to one direction, but vibrations will occur in all directions perpendicular to the direction of travel. We can reduce those vibrational directions to one direction, also known as the privileged direction (Fig. 1.2). We call this plane polarised light. The plane shaped by the vibrational direction and the direction of travel is called the polarisation plane.
Fig. 1.2 Polarisation of light
Polarisation of light can be caused by for example:
1) reflection on an interface between two media with different refractive indices
2) transmission of light through anisotropic media (nicol, Polaroid plate)
3) deformation of isotropic media (e.g. glass, plastic)
A polariser is a crystal or a filter that is used to produce polarised light. In earlier days polarising microscopes contained nicols, built from calcite prisms. Nowadays Polaroid is mainly used. The polarising effect is created through submicroscopic crystals, oriented with their optical axes parallel and in which one of the two vibrational directions of the light passing through is absorbed. These polaroids can be made out of various materials, e.g. from heraphatite crystals or a polyvinyl alcohol layer.
