What's the background picture?

The blog wallpaper image was taken using a polarised light microscope (PLM) and is of crystalline urea. A PLM works much the same as a regular optical microscope except that it uses polarised light to illuminate and image the sample. Light can be thought of being made up of waves vibrating 90 degrees to to the direction of propagation. Polarised light has a single vibration direction, a property which is utilised to analyse the sample. A PLM has two crossed polarisers as seen in Figure 1. The first polariser filters out all vibration directions except one and the second, orientated 90 degrees to the first, blocks the remaining light from reaching the eyepiece of the microscope.

Figure 1 - Polarisation of light in a PLM. (Source: Nikon Microcopy U)

Whilst it may initially seem a little odd filter out all wavelengths of light in light microscopy technique, it can be very useful and provide much information about the sample being analysed. Certain materials can alter the polarisation of light as it passes through them, a property called birefringence. If a birefringent material is placed between the two filters it will alter the polarisation of the light that has passed through the first filter and as a result will not be blocked by the second filter. The material can then be viewed at the eyepiece. There are many different birefringent materials with a wide range of applications, most notably in liquid crystalline displays.




For more PLM images, visit The Wellcome Trust page, the source of the background image.