Zeeman Effect
Zeeman Effect
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In 1896 Zeeman discovered that spectral lines are split up into
components when the source emitting lines is placed in a strong magnetic field.
It is called the Zeeman effect after the name of the discoverer.
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The apparatus used to observe Zeeman effect is shown in the the
following figure:
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The apparatus consists of:
(1) electromagnets capable of producing strong magnetic field with
pole pieces through which holes have been made lengthwise.
(2) Let a discharge tube or sodium vapour lamp emitting radiations
be placed between the pole pieces.
(3) When the spectral lines are viewed axially through the hole in
the pole pieces i.e., parallel to the magnetic field, the line is found to
split up into two components, one having shorter wavelength (higher frequency)
and the other having higher wavelength (shorter frequency) than that of the
original spectral line, which is no longer observable.
(4) The two lines are symmetrically situated around the position of
the original line and the change in wavelength is termed the Zeeman shift (denoted
as dλ). When viewed in a direction perpendicular to the applied field the lines
split up into three, the central one having the same wavelength and frequency
as that of the original line and the other two occupying the same position as
observed earlier.
❒
In order to explain Zeeman effect, let us consider motion of an
electron in a particular orbit corresponding to its permitted angular momentum.
(a) The motion of the electron in an orbit is equivalent to a
current in a loop of wire.
(b) If a current carrying loop of wire be placed in a magnetic
field, it experiences a torque, and energy of the system depends upon the
orientation of the loop with respect to magnetic field.
(c)The correct values of the energies are obtained if the components
of the angular momentum of the electron along the direction of the magnetic
field are restricted to the value:
where m = 0, ± 1, ± 2, ... and so on. Corresponding to these values
of m, a given line splits into as many lines. Hence for each frequency of a
radiation emitted by the atom in the absence of magnetic field, there are
several possible frequencies in the presence of it. This is, in fact, the cause
of Zeeman Effect.
Lorentz’s theory
❒
The shift in the frequency(dλ) for each of the component lines is
given by Lorentz’s theoretically derived equation as
where:
(H) is the strength of magnetic field,
(e) the electronic charge,
(m) the mass of electron,
(c) the velocity of light
(λ) the wavelength of the original line in the absence of magnetic
field.
The equation can also be written as:
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The validity of the above equation can be tested experimentally by
observing the Zeeman shift (dλ) for a given light source of known (λ) (say
D-line of sodium) for a magnetic field of known strength (H) and calculating
the value of (e/m) for the above equation.
❒
Lorentz found that the (e/m) of the electrons found by this method
comes out to be the same as by any other method.
Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolour edition.
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