wave 1                      wave 2
------------------------------------------------------------------------------

  wavelength  lambda            500 nm                      505 nm

                                         7                          7
  wavenumber k           1.256637062 x 10  rad/m    1.244195110 x 10  rad/m

                                         15                         15
  angular freq omega     3.767303135 x 10  rad/s    3.730003104 x 10  rad/s

------------------------------------------------------------------------------

                                    combined wave


  "average" wavelength            502.5 nm


  "envelope" wavelength       101,000   nm         see calculation below

------------------------------------------------------------------------------


  To compute the envelope wavelength, we use the formula

              wavelength  =   2 pi / wavenumber k

but in the combined wave, the envelope term has a wavenumber

                    (k1 - k2) / 2  =   (delta k) / 2

and so

                                  2 pi
    envelope  wavelength  =   --------------
                              (delta k) / 2