well! So sunny! In the first two episodes, some viewers noticed that I had a tan.
In Taiwan, there is never a shortage of sunshine. Advertisements for whitening and sunscreen can be seen everywhere in the market, but why do we get sunburned by the sun? But it seems you’ve never heard of being burned by a fluorescent lamp?
Of course, this is also related to quantum mechanics and is closely related to our topic today.
Before that we talked about the beginning of the quantum concept in history Next, we will explain more how the quantum concept was developed and the story of that person.
photoelectric effect
In the development process of quantum mechanics, the study of the photoelectric effect is 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 certain material, the material emits electrons.
At the end of the 19th century, scientists had already discovered a strange phenomenon: by grazing a negatively charged metal plate with light (especially ultraviolet light) the negative charge of the metal plate disappears. But they did not know the principle behind it at the time, they only speculated that the surrounding gas could help the negatively charged particles to leave the metal plate under ultraviolet light irradiation.
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 mercury gas helped the zinc plate release negative charges. charges spread and ee followed in Particles similar to those discovered from radiation studies two years earlier (1897).
They are tiny particles with a negative charge about a thousand times lighter than hydrogen atoms, which we now call electrons.
In 1902, the German physicist Leonard discovered that even in an empty glass tube, when light above a certain frequency is irradiated, a current passes between the two poles. -lights, and the light will After it is removed, the current will also disappear immediately.
At this point, the concept of the photoelectric effect as we know it is fully formed.
It seems that there is nothing wrong here? However, without using current quantum theory and relying only on physical knowledge at that time, it would be difficult to explain the photoelectric effect perfectly. Because according to traditional theory, the energy of the light should be related to the intensity of the light, not the frequency of the light.
If light transfers energy to electrons and allows them to escape from the metal plate, why does light above a certain frequency have to 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 with ultraviolet rays creates a current. In addition, 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 infrared rays, we find that no current is created at all. But if it is ultraviolet light, even if the intensity is very low, a current will still be created immediately.
Such an incomprehensible photoelectric effect allowed Einstein to turn the entire world of physics upside down in 1905 and establish the foundation of quantum mechanics.
Photoelectric effect explained
To explain the electron-electron effect, Einstein thought that the energy carried by electromagnetic waves is radiated in the form of “light quanta” that carry energy. Referring to Planck’s previous research results, 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 proportionality constant, which is also a constant Planck we introduced.
According to Einstein’s explanation, the higher the frequency of the electromagnetic wave, the greater the energy of the photon. Therefore, as long as the frequency is relatively high, the electrons can gain enough energy to escape from the metal plate and form. electric current on the other hand, if the electromagnetic wave If the frequency is not high enough, the electrons cannot get enough energy to leave the metal plate.
It’s like Dwayne Johnson can knock me down with one punch, but if a weakling hits me a hundred times with one percent of Dwayne Johnson’s strength, even if the total strength is the same, I won’t be knocked down after I to be knocked unconscious. Maybe I felt a dull itch, like I didn’t feel any harm.
When the intensity of the electromagnetic wave is stronger, the number of photons is greater, so the number of electrons that leave the metal plate increases naturally, and the current becomes larger. Just as we have been hit by Dwayne Johnson many times, naturally the injury will be more serious than just one punch.
Although Einstein’s explanation of the electron-electron effect seemed perfect, the view of light quanta was too radical and difficult to be accepted by scientists at the time. Even Planck himself was not happy about it.
For Planck, the fundamental unit energy hν was emitted by a virtual “oscillator”; but for Einstein, the energy of the electromagnetic wave itself was individual light quanta, or what are now called “photons.”
However, electromagnetic waves are waves.
The American physicist Millikan strongly believed that Einstein’s theory was wrong and spent several years doing experimental research on the electron-electron effect.
In 1914, Millikan published the world’s first experimental value of Planck’s constant, which was not far from the now known standard value h = 6.626 × 10-34 Js (joules per second).
In the paper, Millikan tuî-sim-kuann said that the experimental results were surprisingly consistent with Einstein’s quantum theory which was abandoned nine years ago.
Now, 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 electron effect.
Applications for the photoelectric effect
Today, the photoelectric effect has many uses. The common solar power panels in our daily life use a type of photoelectric effect, called the photovoltaic effect After absorbing the energy of photons, the electrons inside the material do not enter to the surrounding area, but moving within the material, creating positive and negative poles to generate current.
Whether you get a sunburn is also related to the energy of the photon.
Sunburn is damage to the skin caused by exposure to high-frequency sunlight, that is, UVB radiation in the ultraviolet rays. When these photons hit the skin, they cause the electrons that make up the bonds in the DNA molecules to escape, causing abnormal changes in the DNA in the skin cells, leading to cell damage and immune responses, swollen, soreness and swelling after sunburn.
Light with a lower frequency, because the photon energy is lower, is less likely to cause damage, which is why we have never heard of sunburn from fluorescent lamps.
Decision
Since the second half of the 17th century, when Huygens and Newton 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, Thomas. Yang used a double-slit interference experiment to demonstrate the wave nature of light By the mid to late 19th century, the conclusion that electromagnetic waves were finally established theoretically and experimentally by Maxwell and Hertz respectively.
After about two hundred years of research and development, the world has come to understand that light is a type of variable.
Suddenly, within a few years, Einstein jumped out and proposed that the energy of light is carried by individual light quanta After passing the experimental test, light became hate it again.
Physicists must admit that light has two characteristics: wave and particle. The characteristic it exhibits depends on the situation, which is called light particle duality.
The concept of quantum light proposed by Einstein in 1905 overturned the traditional view that waves and particles are completely separated also confirmed by subsequent experiments, as well as the energy of light quantities on getting them “measured”, such as electricity costs.
We now know that charge also has a basic unit, which is the amount of charge carried by a single electron.
Although it was later discovered that the quarks that make up the atomic nucleus have fundamental charges of -1/3 and +2/3 units, this did not change because the magnitude of the charges or -continuous, and does not require any amount of electricity.
If you think it’s strange, think about it with the naked eye, we feel that every part of the body is continuous ‘I didn’t see it at all.
The energy of photons and the amount of charge are like this. If you break them down, you will find that they have the most basic units and are not continuous.
In fact, quantum mechanics has been surprising people since its birth. Our next story is about a groundbreaking hypothesis that broke common sense in the development of quantum mechanics. De Broglie won the Nobel Prize in Physics in 5 years for his doctoral thesis.
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