Even when they have the same energy, photons travel different distances within a
material simply based on the probability of their encounter with one or more of the
particles of the matter and the type of
encounter that occurs. Since the probability of
an encounter increases with the distance
traveled, the number of photons reaching a
specific point within the matter decreases
exponentially with distance traveled. As shown
in the graphic to the right, if 1000 photons are
aimed at ten 1 cm layers of a material and
there is a 10% chance of a photon being
attenuated in this layer, then there will be 100
photons attenuated. This leaves 900 photos to
travel into the next layer where 10% of these
photos will be attenuated. By continuing this
progression, the exponential shape of the curve
becomes apparent.