1. Nov 30, 2011

2. Dec 1, 2011

### Simon Bridge

It is to do with how absorption happens.

Let's see if I can give you a simple model:

A photon encounters an atom, it is absorbed if the energy of the photon matches closely enough with the difference in energy between two atomic energy levels. The atom becomes excited for a while, then releases the energy in a series of lower energy jumps or mechanically by jostling adjacent atoms ... so the material heats up.

The higher the energy of the photon, the bigger the energy hit, and for very high energy (eg x-rays or gamma rays) the energy is enough to ionize the atom. Thus even x-rays do not go through everything. In fact, x-ray absorbtion spectroscopy is an important tool.

For a wider discussion see: