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The Zeeman Effect That Helps Measure the Magnetic Field of the Sun

The revolutionary Zeeman Effect won it's discoverers the Nobel Prize in Physics in 1902.

In the late 1800s, a scientist by the name of Pieter Zeeman was burning sodium in his lab when he made a discovery that would propagate through the field of electromagnetics for the rest of history. 

While burning sodium on a bunsen burner, Zeeman was observing the bright D-lines that this element emits - essentially just the spectrum of light rays, similar to the ones that come from the Sun. He decided to subject the burning sodium to a magnetic field and observed that the lines widened and changed. 

Zeeman had discovered that light can be affected by electromagnetic forces. This would go on to be known as the Zeeman Effect. In order to understand the contributions of Pieter Zeeman to the field of physics as well as understand what exactly the Zeeman Effect is, let's dive into more detail. 

What is the Zeeman Effect?

The Zeeman effect, explained simply, is the splitting of a spectral line by the influence of a magnetic field. The spectral lines in burning sodium, like the original experiment carried out by Zeeman, were a little under 600 nm. In this use case, the lines would be split due to subjection to a static magnetic field producing a more and a less energetic line in addition to the original. 

What's exactly causing this interaction is that the static magnetic field excerts a torque on the quantum particles in light, impacting the angular momentum of these particles. 

Understanding this on an even more technical level, the p orbital, a term used to describe the probable places an electron can be found at any given time, has three potential quantum states that it can degenerate into without any loss in energy. However, as we noted before, subjecting light lines to a static magnetic field produces three different energy levels, low, original, and high. 

The Zeeman Effect That Helps Measure the Magnetic Field of the Sun
Magnetic field of the sunspot (left) splits absorption line according to Zeeman effect (right), Source: Hale/Wikimedia Commons

Each quantum state of the p orbital also has a magnetic dipole associated with it, so when the magnetic field comes into contact with the quantum states, it separates them into three different energy levels. 

One of the states elevates the energy of the line, one lowers the energy, and the other remains at the same energy. As these quantum states separate and change energies, they create three different spectral lines of slightly different energy.

Still not really following? Well, summing what I just said up, that's known as the simplest case of the Zeeman effect, otherwise referred to as the Normal Zeeman effect. 

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Coming back into reality for a second, we can understand that the Zeeman effect is the splitting of light waves into different energies based on the forces of a static magnetic field. So how is this useful?

It's useful in areas where we need to measure magnetic field intensities. 

The Zeeman effect correlates the wavelengths of the light waves back to the force of the magnetic field that caused them. This means that with some not so simple math, scientists can back-calculate and determine the size of the original magnetic field that caused the Zeeman effect in the first place. 

The spectral lines of mercury vapor lamp at wavelength 546.1nm, showing anomalous Zeeman effect in the image below.
A. Without a magnetic field
B. With a magnetic field, spectral lines split as transverse Zeeman effect
C. With a magnetic field, split as longitudinal Zeeman effect

The Zeeman Effect That Helps Measure the Magnetic Field of the Sun
The spectral lines were obtained using Fabry-Perot etalon, Source: Warren Leywon/Wikimedia Commons

This is particularly useful in observing and monitoring the magnetic field of the sun and other plasma bodies. This also comes into play in deferent forms of spectroscopy and is even used in MRIs. There's also a chance that birds take advantage of the Zeeman effect to gain a closer monitor on changing magnetic fields. 

Now that I've done my best to explain the Zeeman effect and its uses, let's backtrack to its discovery and look at just what brought about this scientific principle.

How was the Zeeman Effect discovered?

In the 19th century, scientists were first beginning to crack the code and connections between electricity, light, and magnetism. One of the top scientists working on this at the time was a man by the name of Hendrik Lorentz. Lorentz would go on to play a crucial role in the discovery of the Zeeman effect, but he also notably derived the transformation equations of Einstein's theory of special relativity

Lorentz had found that substances emit and absorb light at differing fixed wavelengths. In essence, each substance in existence has a different characteristic spectrum of light that it emits. 

The Zeeman Effect That Helps Measure the Magnetic Field of the Sun
This is a part of the equipment used to observe the Zeeman Effect, Source: NataliiaBashmakova/Wikimedia Commons

In 1986, Pieter Zeeman was studying how light was impacted by magnetic fields. in one of his experiments with burning sodium as the light source, he noticed that the lines in the spectrum of light had split into several lines after subjecting it to a magnetic field. 

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Zeeman was the experimenter in this instance, thus the first to observe and note the effect. Lorentz was Zeeman's mentor at the time and while working together, they realized that the changes in the lines of the light could be explained by the electron theory that Lorentz had formulated. 

Rather than try to explain the electron theory myself, and likely botch it, I'll let Lorentz explain it himself in his Nobel Prize acceptance speech made in 1902.

"When Prof. Zeeman made his discovery, the electron theory was complete in its main features and in a position to interpret the new phenomenon. A man who has peopled the whole world with electrons and made them covibrate with light will not scruple to assume that it is also electrons that vibrate within the particles of an incandescent substance and bring about the emission of light. An oscillating electron constitutes, as it were, a minute Hertzian vibrator; its effect on the surrounding ether is much the same as the effect we have when we take hold of the end of a stretched cord and set up the familiar motion waves in the rope by moving it to and fro. As for the force which causes a change in the vibrations in a magnetic field, this is basically the force, the manifestations of which were first observed by Oersted, when he discovered the effect of a current on a compass needle."

If you haven't already picked up on it, Lorentz and Zeeman won the Nobel Prize for Physics in 1902 for the discovery of the Zeeman Theory. 

Into today, the Zeeman effect continues to help physicists determine the energy level in atoms and determine their angular momentum. It's a great way to study nuclear and other magnetic resonance. Finally, it's used to measure the magnetic fields of stars. 

While all of these fields are likely too complex for us to fully understand, we can admit that the Zeeman effect changed our understanding of magnetic-light interaction forever. 

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