2024-05-05 05:42:00
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GOOD! So sunny! In the first two episodes, some viewers noticed that I was tanned.
In Taiwan, the sun never fails. Advertisements for whitening and sunscreen can be seen everywhere in the market, but why do we get sunburned from the sun? But it seems you’ve never heard of sunburn caused by a fluorescent lamp?
In fact, this is also related to quantum mechanics and is closely related to our topic today.
We discussed previously the starting point of the quantum concept in history. Next, we will explain in more detail how the quantum concept was developed and the story of this man.
photoelectric effect
In the process of development of quantum mechanics, the study of the photoelectric effect constitutes a very important turning point.
The photoelectric effect refers to the phenomenon that when light or electromagnetic waves above a certain frequency are irradiated on a specific material, it emits electrons.
By the end of the 19th century, scientists had already discovered a strange phenomenon: irradiating a negatively charged metal plate with light (especially ultraviolet light) will cause the negative charge on the metal plate to disappear. But at the time, they didn’t know the principle. They only speculated that the surrounding gas might help the negatively charged particles leave the metal plate under the irradiation of ultraviolet light.
So in 1899, the famous British physicist JJ Thomson placed a zinc plate in low-pressure mercury gas and irradiated it with ultraviolet light to study how the gaseous mercury helped the zinc plate release negative charges . charges and followed it in particles similar to those discovered in radiation studies two years earlier (1897).
They are tiny negatively charged particles regarding a thousand times lighter than hydrogen atoms, which we call electrons today.
In 1902, the German physicist Leonard discovered that even in a vacuum glass tube, as long as light above a certain frequency is irradiated, a current passes between the two poles. The size of the current is proportional to the intensity of the light and. the light will also disappear instantly. After removal, the current will disappear instantly.
At this point, the concept of the photoelectric effect as we know it is fully formed.
Looks like there’s nothing wrong here? However, without using current quantum theory and relying solely on the physical knowledge of the time, it would be difficult to perfectly explain the photoelectric effect. Because according to traditional theory, the energy of light should be related to the intensity of light, not the frequency of light.
If light transfers energy to electrons and allows them to escape the metal plate, then why must light above a certain frequency be effective? For example, if we irradiate ultraviolet rays and infrared rays with the same intensity, we will find that only the metal plate irradiated by ultraviolet rays will generate current. Additionally, when the frequency of ultraviolet light is higher, the energy of the electrons is greater.
On the other hand, if we illuminate a metal plate with very high intensity infrared rays, we will find that no current will be generated anyway. But if it is ultraviolet light, even if the intensity is very low, the current will still be generated instantly.
Such an incomprehensible photoelectric effect allowed Einstein to turn the entire world of physics upside down in 1905 and establish the foundations of quantum mechanics.
Photoelectric effect explained
To explain the photoelectric effect, Einstein hypothesized that the energy carried by electromagnetic waves is radiated in the form of energy-carrying “light quanta.” Referring to the results of Planck’s previous research, it is believed that the energy E of the light quantum is proportional to the frequency ν of the electromagnetic wave, written as E = hν. h is the constant of proportionality, which is also the Planck constant that we introduced.
According to Einstein’s interpretation, the higher the frequency of the electromagnetic wave, the greater the energy of the photon. Therefore, as long as the frequency is high to a certain extent, the electrons can obtain enough energy to escape from the metal plate and form. an electric current conversely, if the electromagnetic wave If the frequency is not high enough, the electrons cannot gain enough energy to leave the metal plate.
It’s like Dwayne Johnson can knock me out with one punch, but if a weakling hits me a hundred times with one percent of Dwayne Johnson’s strength, even if the total force is the same, I won’t be not beaten following losing consciousness. I probably felt a persistent itch, as if I didn’t feel any pain.
When the intensity of the electromagnetic wave is stronger, the number of photons is larger, so the number of electrons leaving the metal plate naturally increases and the current becomes larger. Just like we were repeatedly hit by Dwayne Johnson, the injury will naturally be more serious than a simple punch.
Although Einstein’s explanation of the photoelectric effect seemed perfect, the view of light quanta was too radical and difficult for scientists of the time to accept. Even Planck himself wasn’t happy regarding it.
For Planck, the basic unit energy hν was emitted by a virtual “oscillator”, but for Einstein, the energy of the electromagnetic wave itself consisted of individual light quanta, or what are now called “photons”.
However, electromagnetic waves are waves. Intuitively, they propagate infinitely in space. How can they be packets of energy carrying the smallest basic unit of energy?
American physicist Millikan firmly believed that Einstein’s theory was wrong and spent many years conducting experimental research on the photoelectric effect.
In 1914, Millikan published the world’s first experimental value of Planck’s constant, which was not far from the now recognized standard value h=6.626×10-34 Js (joules per second).
In the paper, Millikan said the experimental results were surprisingly consistent with Einstein’s quantum theory that was abandoned nine years ago.
Even if the academic community doesn’t want to believe Einstein, it won’t work. Einstein also won the Nobel Prize in Physics in 1921 for his contribution to the photoelectric effect.
Official portrait of Einstein following receiving the Nobel Prize in Physics in 1921. image/wikimediaApplications of the photoelectric effect
Nowadays, the photoelectric effect has many uses. Common solar panels in our daily life use a type of photoelectric effect, called photovoltaic effect, following absorbing the energy of photons, the electrons inside the material do not radiate into the surrounding space, but move to inside the material, forming positive. and negative poles to generate current.
Whether you get a sunburn is also related to the energy of the photon.
Sunburn is damage caused to the skin by exposure to high-frequency sunlight, that is, UVB rays present in ultraviolet rays. When these photons hit the skin, they cause the electrons that form the bonds in DNA molecules to leak away, causing abnormal changes in the DNA of skin cells, leading to cell damage and immune responses. This is why the skin will appear red. , swollen, painful and inflamed following a sunburn.
Lower frequency light, because the photon energy is lower, is less likely to cause damage, which is why we’ve never heard of sunburn from fluorescent lights.
Conclusion
Since the second half of the 17th century, when Huygens and Newton each proposed the wave theory and the particle theory of light, people have focused on the big question of whether light is a wave or a particle in the early 19th century. century, Thomas. Yang used a double-slit interference experiment to show the wave nature of light. By the mid-to-late 19th century, the conclusion that light belonged to electromagnetic waves was finally established theoretically and experimentally by Maxwell and Hertz respectively.
After regarding two hundred years of research and development, the world has understood that light is a kind of fluctuation.
Unexpectedly, a few years later, Einstein took the plunge and argued that light energy was carried by individual light quanta. After passing the experimental test, the light became a particle once more.
Physicists must recognize that light has two properties: wave and particle. The characteristic it will exhibit depends on the situation, called wave-particle duality of light.
The quantum concept of light proposed by Einstein in 1905 disrupted the traditional view that waves and particles are completely separate. The interpretation of light energy quantification was also confirmed by experiments. After that, in addition to physical energy, there are other physical phenomena. quantities. Found “quantized”, like electric charges.
We now know that charge also has a base unit, which is the size of the charge carried by a single electron.
Although it was later discovered that the quarks that make up the atomic nucleus have base charges of -1/3 and +2/3 units, this does not change the fact that the size of the charge is discontinuous and does not requires no quantity of units. electricity.
If you think that’s strange, think regarding it. With the naked eye, we feel that each part of the body is continuous. But actually, on a microscopic scale, the body is also made up of very small atoms and molecules, but we can do this. I don’t see it at all. It was only following I came out that I felt it was continuous.
Photon energy and charge size are actually so if you break them down you will find that they have the most basic units and are not continuous.
In fact, quantum mechanics has continued to surprise people since its birth. It’s like an elusive cat suddenly bursting into the world of physics. Our next story concerns a revolutionary hypothesis that shattered common sense in the development of quantum mechanics. The proponent of the hypothesis initially majored in history and law in college and had a bachelor’s degree in history, but later changed his major to physics. De Broglie won the Nobel Prize in Physics within 5 years for his doctoral thesis.
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