[Wangs, p.204, l.-17-l.-6]
treats a volume current density and a surface current density as parallel
concepts, while [Fan, p. 85, l.6-l.20] treats a surface current density as the limit case
of a volume current density. Therefore, if we want to visualize a surface
current density as a volume current density, all we have to do is add a tiny
thickness to the surface in [Wangs, p.205, Fig. 12-6 (b)].
A charge density that is zero in 3-dim can be nonzero in 2-dim. If this is the case, how do we define the surface charge density? [Born, p.5, (18)] gives a proper definition. In contrast,
both [Wangs, (2-16)] and [Wangs, (12-10)] are inadequate. The former fails to
relate it to the 3-dim charge density and the latter fails to specify that
dh→0. [Wangs,
(9-24)] gives unnecessary details, which make the definition more artificial and confusing. For
example, it is hard to visualize how r is
related to the surface charge density except through its contribution to the total
charge.
When we discuss the law of refraction, what does the theoretical expression
l0®0
mean [Born, p.125, l.-10]?
Answer. In practice, we mean that the wavelength is relatively small
when compared with
the radii of curvature of the incident wave and of the boundary surface [Born,
p.125, l.-9-l.-7].