The physicist Planck once said: "The new scientific facts are accepted not because the opponents are persuaded, but because the opponents are eventually dead ..." ”
Hearing this, we all felt that Planck was a man who was very open to new ideas, but he did not.
In order to solve the problem of blackbody radiation, he himself has put forward the assumption that blackbody radiation energy is "one by one", but he himself can't believe that this is true. This hypothesis of his is only in the mathematical sense, not a physical idea. (This is the question of blackbody radiation: blackbody radiation from the birth of quantum theory: the real revolution is that even the revolutionaries themselves are against it)
Through his hypothesis, Einstein was inspired to boldly propose that to solve the problem of photoelectric effect, light must also be "one grain at a time". At this time, Planck was still confused, and he thought that Einstein's "joke" had "widened."
Einstein and Planck
What is the photoelectric effect? Simply put, when light shines on metal, it can excite the phenomenon of electrons. This phenomenon was discovered by Hertz in 1887.
When light shines on metal, it will excite electrons one by one, but not all light can shoot electrons out of metal. In experiments, physicists have found that different colors of light can hit electrons, and some can't. For example, red light in visible light cannot shoot electrons from metal, but blue light or ultraviolet light in invisible light can.
No matter how strong the red light is used to illuminate the metal for a long time, the electrons cannot be excited, and as soon as the ultraviolet rays are irradiated, the electrons are immediately excited. That's weird!
Maxwell's theory of electromagnetism in classical physics firmly believes that light is an electromagnetic wave, and the energy it emits is continuous. According to electromagnetic theory, whether electrons can be excited is related to the intensity of light, and the intensity of light can accumulate more and more energy by continuously illuminating metals, so why can visible light not be hit no matter how long it is irradiated? On the contrary, light similar to ultraviolet light, no matter how weak, can excite electrons as long as it is irradiated on metal.
This photoelectric effect phenomenon cannot be explained by classical physical theory.
Einstein once used the assumption that the speed of light is invariant and pioneered the special theory of relativity. Now, he has come to think again, and he proposes that if the energy of light is regarded as a copy, it can explain the problem of the photoelectric effect.
So in 1905, he published a paper, "An Illuminating View of the Production and Transformation of Light", which clearly stated his view: light is a "wave packet", and the minimum energy of this "wave packet" is E = hν, h is Planck's constant, and ν is the frequency of light. The greater the frequency, the greater the energy.
When light hits the surface of a metal, the electron absorbs a fraction of the light quantum, and the blow to the metal depends mainly on the energy of the individual photons rather than the intensity of the light. Therefore, whether the electron can be excited is only related to the frequency of the light, and it is not related to the intensity of the light, the intensity of the light is only the density of the photon flow.
Because the energy of a high-frequency light is large enough, it can shoot out photons. It's like, we shoot a brick wall with a toy gun, no matter how long you strafe, the wall will not collapse, but with a strong impact shell, a cannon can blast it.
Einstein's theory of light quanta was not until 1926 that the American physicist Lewis replaced it with the term we commonly use today - "photon".
Einstein's paper on the properties of light eventually won the 1922 Nobel Prize in Physics, the only Nobel Prize of Einstein's lifetime.
You know, that year, that year, in 1905, Einstein published a total of 6 papers, of which there were special relativity, there were real hammers to prove the existence of atoms about the thermal motion of molecules, and there were also about the law of conservation of mass energy... From now on, all articles can reach the level of winning the Nobel Prize. However, because scientists still had doubts about the theory of relativity at that time, they did not give the prize, and the Nobel Prize Committee stipulated that a person could only receive the Nobel Prize once in the same field. So Einstein can only take this Nobel Prize about light.
Einstein in 1905
Einstein's quantum theory of light, he not only predicted the particle properties of light. In 1909, he also proposed that photons should have momentum. In a later paper , he gave the formula for the momentum of photons as : p = h / λ ; where p is the momentum of each photon , λ is the wavelength of light.
He applied the mass-energy equation in special relativity to photons, resulting in a photon kinetic energy of E=mc^2. In his light quantum theory, the kinetic energy of photons is: E = hν = hc / λ. When the two are combined, p=mc=h/λ is obtained. where c is the speed of light, which is both the speed at which photons move and the speed at which electromagnetic waves travel.
Students who are allergic to the formula can directly understand the following text:
Einstein unified particles and waves through these formulas: the energy and momentum of a particle are calculated by the frequency and wavelength of the wave, that is, Einstein gave light to both the properties of the particle and the wave. So he asserted: Light has wave-particle duality!
The discovery of wave-particle duality not only ends the dispute that has lasted for more than 300 years, but also unveils the mystery of the quantum world, which is actually the strange "goblin" in the quantum world that drove many scientists, including Albert Einstein, crazy. (For this centuries-long dispute, see: Quantum Mechanics Prequel: Whether Light Is a Wave or a Particle Spanning 300 Years, What exactly is the contention?) ）
Planck hypothesized that the energy radiated by black bodies was a part, and Einstein assumed that the energy of light incident on an object was a part, and although the two were different, they proposed a completely new concept, that is, the concept of quantization of energy, and since then physics in the 21st century has created a revolution in quantum mechanics.
This groundbreaking insight and thought of Planck and Einstein now seems natural and obvious to us. It's not that easy.
Einstein's theory of the quantum of light was initially not recognized by the scientific community, and many physicists thought that this hypothesis was "completely untenable", and even Planck himself could not believe it. Isn't it a joke that the idea of light as a wave in classical physics is rooted in people's brains, and then suddenly saying that light is a particle?
In order to prove that Einstein's quantum theory of light was wrong, Millikan repeatedly did many experiments, but in 1951 he confirmed that Einstein's theory of light quantum was correct.
Even when Einstein won the Nobel Prize in 1922 for the photoelectric effect, the prize committee tried to avoid acknowledging the particle nature of photons when awarding the prize, and only mentioned the term light quantum when describing the emission and absorption process. They argue that the most important application and proof of Einstein's theory of the photoelectric effect comes from bohr's use in models of atomic structure, thus explaining the problem of spectral lines.
It was not until 1923, 18 years after Einstein's theory of the photoelectric effect was proposed. Compton observed in the experiment that some of the X-rays would deviate from the original direction after being scattered, and the wavelength became longer.
Compton
This cannot be explained by classical physical theory. The explanation proposed by Compton is that this scattering process can be seen as the collision of two particles, one is a free electron and the other is a photon. That part of the wavelength-lengthening rays is caused by the collision of photons and electrons.
The photon carries not only energy, but also momentum, and when it collides with an electron, it transfers a portion of its own energy to the electron. In this way, the energy of the photon will drop, according to the formula: E = hν, the energy E drops, the frequency ν also drops, the frequency is inversely proportional to the wavelength, so the wavelength becomes larger. This is known as the Compton effect.
God created light, Einstein pointed out what is the nature of light, and Compton was the first person to "see" light in the true sense. At this point, people truly recognized Einstein's quantum theory of light.
Einstein's biographer once wrote: "From 1905 to 1923, Einstein was the only one, or almost the only one, who took the quantum of light seriously. ”
How did Bohr build his atomic model using Planck and Einstein's theory of quantization of energy? Let's talk about it next time ~ please pay attention to @sauce to talk about science.