Zeeman effect in neon using the home-made etalon

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thesmiths
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Zeeman effect in neon using the home-made etalon

Post by thesmiths »

To conclude our experiments with our home-made Fabry-Perot etalon, we decided to revisit neon. The issue with neon is that it has a lot of bright lines spaced quite close together in the yellow/red part of the spectrum. This required using a narrower slit (1mm vs 2mm for helium and mercury) and a more dispersive grating (1200 vs 600 lines/mm). The collimating lens this time was a 135mm f5.6 enlarging lens and the camera lens was a 100mm f4 macro bellows lens. A 25mm f3 planoconvex lens was used to concentrate light from the neon discharge tube onto the slit.

The image below is the spectrum of neon taken with our home-made grism spectrometer, taken with a Canon 100D.
neon spectrum
neon spectrum
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The box in the image above corresponds to the box in the Zeeman spectrum below, which was taken with the same camera (without a polariser).
neon Zeeman spectrum
neon Zeeman spectrum
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The bright yellow line on the left of the box shows "normal" Zeeman splitting, while many of the other lines show the "anomalous" Zeeman effect with a much more complicated structure. The complex structure of the "anomalous" lines could not be fully explained when they were first observed.

With so many of the lines in the red, a monochrome camera is more useful. The GIF below, taken with the ASI 174MM camera, shows the splitting with a linear polariser at 45, 0 and 90 degrees with respect to the magnetic field. The field of view corresponds to the box in the images above. The lines correspond to the wavelengths (in Angstroms): 5853 5882 5945 5976 6030 6075 6096 6143 6164 6217 6267 6305 6335 6383 6402
monochrome Zeeman 1
monochrome Zeeman 1
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The following image is displaced to the right and captures the rest of the lines in the colour Zeeman spectrum as well as two lines in the infrared. The lines correspond to the wavelengths (in Angstroms): 6507 6533 6599 6678 6717 6930 7033
monochrome Zeeman 2
monochrome Zeeman 2
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The initial discovery, in the first years of the 20th century, of complex Zeeman splitting in atoms like neon was one of the motivating factors for the development of quantum mechanics in the 1920s and 30s (in particular the concept of electron spin).


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Re: Zeeman effect in neon using the home-made etalon

Post by marktownley »

Enjoy reading about these experiments! Great stuff.


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thesmiths
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Re: Zeeman effect in neon using the home-made etalon

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To complete the experiment, we made the effort to look at the longitudinal Zeeman effect. This required using a permanent magnet with a hole down the middle. When you look through the hole, the magnetic field lines are (more or less) parallel to the line of sight. This also required drilling a small hole through our steel yoke that supports the pair of magnets and confines the stray field. When you look perpendicular (as we did previously), the light is linearly polarised. When you look longitudinally, the light comes out circularly polarised. To observe this, we bought a small sheet of quarter wave retarding plastic (from Edmund Optics). We positioned this between our collimating lens and etalon. The linear polariser was on our camera lens and when this was rotated from +45 degrees to -45 degrees, the two filters together acted as a circularly polarising filter. At zero degrees, both polarisations are let through (so equivalent to no filter).

The GIF below shows the same neon lines as above but now looks at the two circular polarisations. So the three frames of the GIF are left, right and both polarisations. I won't go into all the details about how this GIF differs from the previous one. But it was the prediction and confirmation of the different light polarisations that won Zeeman and Lorentz the 1902 Nobel Prize in Physics (the second one awarded).
neon longitudinal Zeeman
neon longitudinal Zeeman
20210324_033646.gif (3.44 MiB) Viewed 79 times


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