The dielectric susceptibility of helium at one atmosphere pressure and 
is
. Find the atomic polarizability 
of helium.
[Recall that the dielectric susceptibility is defined from the permttivity by
.
Recall also the fundamental relation between refractive index and polarization:
Helium is an ideal gas so the pressure is 
]
SOLUTION:
We assume the refractive index is purely real number, since the susceptibilty is said
to be real.
The susceptibility relates to the refractive index and hence the polarizability via the
result about the phase velocity of radiation.
For an ideal gas the pressure is where Boltzmann's constant is 
and T is the Kelvin temperature =Celsius+273.
(using atmospheric pressure =101kPa).
