Looking down an arbitrary line of sight of travel along which an individual photon could travel, then typically a molecule will be seen head on, as it were, and this molecule would be at some particular depth. Assuming a random collection of molecules within the material, it is of course not a certainty that a molecule will be seen head on for any arbitrarily chosen line of sight, but as the depth increases there is more and more chance that such a molecule will arise. In The Ubiquitous Exponential Laws II - Radioactive Decay , to appear in the next issue of Plus , Ian Garbett shows how radioactive decay obeys an exponential law, and explains how we can use this fact to date artefacts such as the Dead Sea Scrolls. He graduated in 1977 with a BSc Honours in Applied Physics from the University of Lancaster, and obtained an MSc in Medical Physics from the University of Leeds in 1987. He is interested in various theoretical aspects of radiation and radiological physics, with an interest in mathematical modelling in general.
Current research involves a theoretical description of X-ray beam spectra.