Q:  What happens to this function when wavelength becomes very small?

         In this case, the term on the bottom of the fraction approaches

                       (lambda k T / h c)  
                      e

         which grows very large, very quickly, as lambda decreases.
         Since this is on the bottom of the fraction, this means
         that the entire function behaves like

                       - lambda
                      e

         and that explains the high-energy, left-hand side of the graph.


  Q:  What happens to this function when wavelength becomes very large?
  
         In this case, one can use the binomial expansion to re-write
         the exponential as

                   - hc / lambda kT                       h c
                  e                    = approx  1  -  ----------
                                                       lambda k T


         which means that the bottom of the fraction becomes

                         h c                       h c
                1  -  ------------  -  1  =    -------------
                      lambda k T                lambda k T


         and so the entire function becomes


                     h c^2 / lambda^5           c k T
                  ----------------------  =  --------------
                     h c / lambda k T         lambda^4 

       
         which clearly is a power law, with slope -4.