Investigating Things to Gain Knowledge 02 - Beat Frequency of Two Columns of Light
I have previously posted a post titled "Investigating Things to Gain Knowledge 01 - Beat Frequency". Link
https://bbs.eeworld.com.cn/thread-579258-1-1.html In that post I said: ———————————————————— The "beat" phenomenon is very common in machinery and circuits. The so-called "beat" refers to a lower frequency amplitude signal synthesized by two sinusoidal signals with similar frequencies. The signal here can be an electrical signal (electromagnetic vibration, usually called electromagnetic oscillation) or a sound signal (mechanical vibration). ———————————————————— Sound is a wave, and it is easy to detect the "beat" phenomenon produced by the sound emitted by two sound sources with very close frequencies. This has been described in detail in the previous post. However, light is an electromagnetic wave, and is also a wave. Two sound waves with similar frequencies can produce a "beating" phenomenon, and two light waves with very close frequencies should also produce a "beating" phenomenon. However, for a long time, physicists have not been able to discover the "beating" phenomenon of light. The main reason is that it is difficult to find two beams of light with very close frequencies. In 1896 (the 22nd year of the Guangxu period of the Qing Dynasty), Dutch physicist Pieter Zeeman used a concave Rowland grating with a radius of about 3 meters to observe the spectrum of a sodium flame in a magnetic field. He found that the D spectrum line of sodium seemed to be broadened. Subsequent more detailed spectral analysis showed that the spectrum line was actually split into several spectrum lines that were almost close together. Later generations called the phenomenon of the splitting of spectrum lines produced by atomic light emission in a magnetic field the Zeeman effect.
Figure 01 Peter Zeeman
Shortly afterwards, Zeeman's teacher, Dutch physicist Hendrik Antoon Lorentz, applied classical electromagnetic theory to explain the phenomenon of spectrum line splitting, also known as the Zeeman effect.
Figure 02 Hendrik Anton Lorentz
1896 was truly a year of great discoveries in physics. In this year, Professor of Physics at the University of Würzburg in Germany, Roentgen, announced the discovery of X-rays (Roentgen's experiment to discover X-rays was in November of the previous year). In the same year, French physicist Becquerel discovered the radioactivity of uranium. The discovery of X-rays, the discovery of uranium radioactivity, and the discovery of spectrum line splitting in a magnetic field have won the first three Nobel Prizes in Physics. The first Nobel Prize in Physics was awarded to Roentgen in 1901, the second Nobel Prize in Physics was awarded to Zeeman and Lorentz in 1902, and the third Nobel Prize in Physics was awarded to Becquerel and the French physicist Curie in 1903. The Zeeman effect has played a very important role in the development of physics. The Zeeman effect confirms that atoms have magnetic moments and that the spatial orientation of atomic magnetic moments is quantized. The Zeeman effect also leads to the hypothesis of electron spin. In astrophysics, how do we know that sunspots have strong magnetic fields? Obviously, it is impossible to measure near sunspots, and it all depends on the Zeeman effect in the sunspot spectrum. In 1955, almost 60 years after Zeeman discovered that atomic light in a magnetic field would produce spectral line splitting, American physicists Forrest, Gudmundsson and Johnson conducted an experiment. They placed a mercury vapor discharge tube in a magnetic field and made the mercury vapor discharge tube glow. Due to the Zeeman effect, a green spectrum line in the mercury spectrum splits into two spectrum lines with very close frequencies, and the difference between the two frequencies is proportional to the strength of the magnetic field. The average frequency of the two green spectrum lines is 5.49*10^14Hz, and the difference in frequency between the two spectrum lines is obtained by adjusting the magnetic field, and is measured by optical methods to be 10^10Hz. 10^10Hz (10 to the 10th power Hz, that is, 10GHz) is a typical "radar" frequency or "microwave" frequency, which can be measured by electronic methods. The three physicists projected the light of the two mercury green spectrum lines onto a photodetector (actually a photomultiplier tube, a special vacuum tube), and the current output by this photodetector showed a 10^10Hz component. In other words, this photodetector detected the "beat" produced by two light sources with a frequency difference of 10^10Hz. This is really a beautiful physics experiment. Let's think about it more deeply: What does the photodetector (actually a photomultiplier tube) detect? The output current of the photomultiplier tube increases with the increase of the light flux incident on the photomultiplier tube. In other words, the photomultiplier tube detects the light flux, and the incident light flux is the radiation energy passing through the photomultiplier tube incident window per unit time, that is, the power of light. Therefore, the output current of the photomultiplier tube contains a 10^10Hz component, which means that the light power incident on the photomultiplier tube is changing, and the frequency of the light power change is 10^10Hz. This is the physical meaning of the "beat" produced by the two trains of light waves - the frequency of light power change. ——————————————————
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