In the text of the home assignment 4.1, there was a misprint concerning the units of tex:\displaystyle{{n}_{{2}}}  (a minus sign in the exponential)!

The correct dimensions are: tex:\displaystyle{{{m}}^{{2}}\cdot{{W}}^{{-{{1}}}}}    (i.e. not m^-2 W^-1)

I do apologise for the misprint. 

(One more good reason to always perform dimensionality checks...)