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Normal and Anomalous Zeeman Effect

Normal and Anomalous Zeeman Effect: Zeeman observed that when an atom (or light source) is placed in an external magnetic field, the atomic spectral lines it emits are splits into several polarised components. This effect of magnetic field on atomic spectral lines is called Zeeman effect.

Normal and Anomalous Zeeman Effect

A single spectral line seen normal to the field is split into three plain polarised components, a central unshifted line with the electric field vector vibrating parallel to the field called π components and two other lines equally displaced on either side of the central component with electric field vector perpendicular to the field called σ-component. 
This is called normal triplet, and the effect is called Normal Zeeman Effect.
The fine structure components of multiple spectral lines, however, shows a complex Zeeman pattern. For example, the D1 and D2 components of sodium yellow doublet give 4 and 6 lines respectively in the Zeeman pattern. This is Anomalous Zeeman effect. The Zeeman Splitting is smaller than fine-structure splitting.
Normal and  Anomalous Zeeman Effect

Experimental Set-up for studying Zeeman Effect

The experimental arrangement for observing Zeeman effect should be of high
resolving power and large light-gathering power. One arrangement using a
constant-deviation prism spectrometer is shown in Fig.2.
T is a neon discharge tube (line source) with its capillary part placed between the
poles of an electromagnet, and C is a condensing lens.
The light from the capillary part of the tube is condensed by the lens C on the slit of the collimator of the spectrometer. A Lummer-Gehrcke plate (a high-resolving optical device) is placed between the collimator and the constant-deviation prism.
The light emerging from the prism at right angles to its initial direction is received in the telescope fitted with a micrometer eyepiece.
Normal and  Anomalous Zeeman Effect

Adjustment and Procedure :

 
  • In the beginning, the current in the electromagnet is kept off. The condensing lens C, the L-G plate and the micrometer eyepiece are removed. The slit of the collimator is kept fairly wide open. On looking through the telescope. The images of the pole-pieces and the neon tube are seen. The pole-pieces and the neon tube are so adjusted that the image of the pole-pieces appears central in the field of view. Also, the neon tube is symmetrically between the pole-pieces.
  • The condensing lens C is placed between the electromagnet and the slit of the collimator. It is so, such that its aperture is fully illuminated and the image of the aperture fills the field of view. The micrometer eyepiece is put in position and focussed on the crosswise. Now, on looking through it, a bright spectrum of neon light is seen.
  • The L-G plate mounted on its stand is placed in position on the spectrograph. Now, on looking through the eyepiece, each spectral line show a few orders, as shown in Fig. 3 (a). The screws permitting adjustments- of the plate in various directions are finely rotated to obtain a bright and sharp fringe-system. Have a look at these pictures below:

         Pictures of Spectral Lines:

  • The singlet yellow line ( λ= 5852A°) of the spectrum is recognized, and by setting the cross-wire on successive orders (fringes) the readings of the micrometer are taken.
  • The electromagnet is now switched on. Next, the current in it is adjusted so as to give a field of about 4000 gauss. Each order, say a and b, is split into three components. First one undisplaced component and two symmetrically displaced components on either side of it (Fig 3 b). The cross-wire is set, turn by turn, on the displaced components and the reading of the micrometer are taken.
  • At last, the measurements are repeated for different fields, such as 7000 gauss and 10,000 gauss; and also for singlet red line (λ=6266A°) of the neon spectrum.

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