dust_grain.py -- photons scattered by a sunlit dust grain (radius 0.5 um, 1 us of sunlight at 1 AU)
solar photon flux 6.304e+21 m^-2 s^-1;  photons crossing the geometric cross-section in 1 us: 4.951e+03
size parameter ka over the visible band: 4.5 to 7.9;  ka = 3.4 at lambda = 0.92 um
     index m  N_scat (Mie)  N_scat (dipole)  dipole / Mie  dipole/Mie at ka>=3.4  RZ 1e8 / N_scat
   (1.33+0j)     9.025e+03        4.189e+05          46.4          7 to 11177                11080
    (1.5+0j)     1.202e+04        8.706e+05          72.4          8 to 21784                 8317
 (1.7+0.01j)     1.237e+04        1.504e+06         121.6         13 to 42756                 8086
if '1 um' is the radius (a = 1 um):
    (1.5+0j)     5.239e+04        5.572e+07        1063.5                                    1909
 (1.7+0.01j)     4.635e+04        9.624e+07        2076.4                                    2157
checks: Q_sca(x=0.05, m=1.5) Mie/Rayleigh = 1.000176 (-> 1);  Q_sca(x=400, m=1.5) = 2.047 (-> 2)
about 1e4 (1 um diameter) to 5e4 (1 um radius) photons scatter, so the grain carries at most that many records, not ~1e8