In the ¯rst lecture, we stated that the energy den-
sity of radiation per unit frequency interval u(º)
for black-body radiation is described by the Planck
formula (Figure 10.1),
u(º) dº =
8¼hº3
c3
1
(ehº=kT ¡ 1)
dº (10.1)
where Planck's constant, h = 6:626 £ 10¡34 J s.
In this lecture, we demonstrate why quantum con-
cepts are necessary to account for this formula. The
programme to derive this formula is as follows.
² First, we consider the properties of waves in
a box and work out an expression for the ra-
diation spectrum in thermal equilibrium at
temperature T.