Zeeman effect is a phenomenon of splitting a spectral line into a number of component lines with the help of a strong magnetic field. It was first observed by the Dutch physicist Pieter Zeeman in 1896. He observed that whenever a light source is placed in a magnetic field, then each of the lines of its emission spectrum gets split into different component lines. In 1902, Hendrik Antoon Lorentz and Pieter Zeeman received a Nobel prize for their discovery. In a nutshell, the Zeeman effect is a method of visualising the spectrum of an atom in the presence of an external magnetic field. The spectral lines of an atom get split into two or more component lines when the atom is placed under the influence of an external magnetic field.
A light source is actually the release of energy when an excited electron returns from a higher energy state known as the excited state to a lower energy state called the ground state due to its instability in the excited state. If the light source is placed in a strong magnetic field, then the spectrum lines get split into three component lines, while if the light source is placed in a weak magnetic field, then the spectrum gets split into more than three component lines. Originally, in absence of the magnetic field, the atomic energies depend on the principal quantum number n, and the emission occurs at a single wavelength. The splitting observed due to the Zeeman effect represents the interaction between the applied magnetic field and the magnetic dipole moment associated with the orbital angular momentum.
The frequencies for the split component lines can be predicted by using the following formula,
v= v0 ± eB/4πmc
Here, ν0 is the unshifted line frequency, e is the charge of an electron, B is the applied magnetic field strength, m is the mass of an electron, and c is the speed of light.
The vector components of the electron acceleration perpendicular to the line of sight produce the electromagnetic radiations along the line of sight. Since the electrons accelerate in a circular direction, they emit circularly polarized radiation. The right circularly polarized radiations have greater frequency, while the left circularly polarized radiations have lower frequency because of the negative charge on the electrons.
Types of Zeeman Effect
1. Normal Zeeman Effect
If the net spin of the optically active electron of an atom is equal to zero, then it exhibits a normal Zeeman effect. The normal Zeeman effect splits the spectral line of an atom into three major component lines. The explanation of the normal Zeeman effect is available in both Classical and quantum mechanics. In other words, when the splitting of a single spectral line of an atom into three component lines due to the action of the magnetic field is observed, then such a phenomenon is known as the normal Zeeman effect. For instance, if you consider the spectrum of a hydrogen atom, it consists of certain five spectral lines, namely, Lyman, Balmer, Paschen, Bracket, and Pfound. The Balmar line of the hydrogen spectrum lies in the visible range and can be observed when the electron jumps from n=3 to n=2 state. When the Balmar line of hydrogen atoms is placed under the influence of a magnetic field, it gets split into three component lines. The middle line is known as the pi component, while the other two lines, located on either side of the pi component line, denote the sigma components. The sigma components are equidistant from the pi component line. The vibration of the electric vector of the pi component of a spectral line is parallel to the applied magnetic field, while the vibration of the electric vector of the sigma component is perpendicular to the external magnetic field.
2. Anomalous Zeeman Effect
The net spin of the optically active electron of an atom exhibiting anomalous Zeeman effect is not equal to zero. The anomalous Zeeman effect causes the atomic spectral lines to get split into more than three component lines. This effect can be explained only with the help of quantum mechanics. The concept of spinning of electrons was not known when the Zeeman effect was discovered, which is why there was no perfect explanation available for the splitting of atomic spectral lines into multiple component lines at that time. Hence, the new elaborated theory given by Thomas Preston in 1897 was named the anomalous Zeeman effect. In other words, the phenomenon of splitting the fine structure of an atom into its components by placing it under the influence of an external magnetic field is known as the anomalous Zeeman effect. For instance, if you observe the fine structure of a sodium atom, it consists of two spectral lines, namely D1 and D2 lines. The wavelengths corresponding to both the spectral lines are 5896 A° and 5890 A° respectively. When such a fine structure of sodium atom is placed under a magnetic field, the D1 spectral line gets split into four component lines, two of which are the pi component lines, and the other two are sigma component lines. In a similar manner, the D2 spectral line splits into six component lines out of which two are pi component lines, while four are sigma component lines. In the case of the anomalous Zeeman effect, the distance between the component lines may or may not be the same, i.e., the component lines are not necessarily equidistant.
Applications of Zeeman Effect
1. Zeeman effect helps the physicists to determine the energy levels of an atom and to study their angular momenta.
2. It also expands the scope of studying atomic nuclei and phenomena like electron paramagnetic resonance.
3. Zeeman effect is also used in the field of astronomy to study the magnetic field of the sun and other stars.
4. Zeeman effect finds its prime application in various spectroscopy techniques such as nuclear magnetic resonance spectroscopy, electron spin resonance spectroscopy, Mössbauer spectroscopy, etc.
5. Some medical imaging techniques such as magnetic resonance imaging (MRI) also make use of the Zeeman effect.
6. Zeeman effect is also known to exhibit polarization effects. One of the best real-life examples of which are polarized sunglasses. The purpose of polarized sunglasses is to suppress ambient glare.