e H dµ
½ --- ---- ---------,
m v0 d[lambda]
where [lambda] is the wave length. This is equivalent to (2) if µ is
given by (1). He has shown that this expression is in good agreement
with experiment. The sign of r depends on the sign of e, hence the
rotation due to negative ions would be opposite to that for positive.
For the great majority of substances the direction of rotation is that
corresponding to the negation ion. We see from the equations that the
rotation is very large for such a value of p as makes P = 0: this
value corresponds to a free period of the ions, so that the rotation
ought to be very large in the neighbourhood of an absorption band.
This has been verified for sodium vapour by Macaluso and Corbino.[43]
If plane-polarized light falls normally on a plane face of the medium
containing the ions, then if the electric force in the incident wave
is parallel to x and is equal to the real part of A[epsilon]^[l(pt -
qz)], if the reflected beam in which the electric force is parallel to
x is represented by B[epsilon]^[l(pt + qz)] and the reflected beam in
which the electric force is parallel to the axis of y by
C[epsilon]^[l(pt + qz)], then the conditions that the magnetic force
parallel to the surface is continuous, and that the electric forces
parallel to the surface in the air are continuous with Y0, X0 in the
medium, give
A B [iota]C
----------------- = ----------- = ----------
(q + q1) (q + q2) (q² - q1q2) q(q2 - q1)
or approximately, since q1 and q2 are nearly equal,
[iota]C q(q2 - q1) (µ² - 1)pH
------- = ---------- = ------------.
B q² - q1² 4[pi]µne V0²
Thus in transparent bodies for which µ is real, C and B differ in
phase by [pi]/2, and the reflected light is elliptically polarized,
the major axis of the ellipse being in the plane of polarization of
the incident light, so that in this case there is no rotation, but
only elliptic polarization; when there is strong absorption so that µ
contains an imaginary term, C/B will contain a real part so that the
reflected light will be elliptically polarized, but the major axis is
no longer in the plane of polarization of the incident light; we
should thus have a rotation of the plane of polarization superposed on
the elliptic polarization.
_Zeeman's Effect._--Faraday, after discovering the effect of a magnetic
field on the plane of polarization of light, made numerous experiments
to see if such a field influenced the nature of the light emitted by a
luminous body, but without success. In 1885 Fievez,[44] a Belgian
physicist, noticed that the spectrum of a sodium flame was changed
slightly in appearance by a magnetic field; but his observation does not
seem to have attracted much attention, and was probably ascribed to
secondary effects. In 1896 Zeeman[45] saw a distinct broadening of the
lines of lithium and sodium when the flames containing salts of these
metals were between the poles of a powerful electromagnet; following up
this observation, he obtained some exceedingly remarkable and
interesting results, of which those observed with the blue-green cadmium
line may be taken as typical. He found that in a strong magnetic field,
when the lines of force are parallel to the direction of propagation of
the light, the line is split up into a doublet, the constituents of
which are on opposite sides of the undisturbed position of the line, and
that the light in the constituents of this doublet is circularly
polarized, the rotation in the two lines being in opposite directions.
When the magnetic force is at right angles to the direction of
propagation of the light, the line is resolved into a triplet, of which
the middle line occupies the same position as the undisturbed line; all
the constituents of this triplet are plane-polarized, the plane of
polarization of the middle line being at right angles to the magnetic
force, while the outside lines are polarized on a plane parallel to the
lines of magnetic force. A great deal of light is thrown on this
phenomenon by the following considerations due to H. A. Lorentz.[46]
Let us consider an ion attracted to a centre of force by a force
proportional to the distance, and acted on by a magnetic force
parallel to the axis of z: then if m is the mass of the particle and e
its charge, the equations of motion are
d²x dy
m --- + ax = -He --;
dt² dt
d²y dx
m --- + ay = He --;
dt² dt
d²z
m --- + ax = 0.
dt²