This distribution can describe a number of physical phenomena, such as the
time **t** for a radioactive nucleus to decay, or the time **x** for a component
to fail, or the distance **z** a photon travels in the atmosphere before
suffering a collision with a water molecule.
The exponential distribution is characterized by the single parameter
, and one can easily show that the mean and variance
for the exponential distribution are given by:

Figure 6 illustrates the exponential distribution. Note that the standard deviation of the exponential distribution is