Light quantum theory and photoelectric effect

Light Quantum

When the photoelectric effect of light occurs, the emitted electrons are divided into one copy, and each one is called a light quantum, also called photoelectron.

Photoelectric effect

When light strikes a substance, it causes the electrical properties of the substance to change. This phenomenon is called the photoelectric effect. The wavelength of light that has just undergone the photoelectric effect is called the limit wavelength, and the frequency of light is called the limit frequency. The photoelectric effect occurs only when the wavelength is less than (including equal to) the limiting wavelength. The value of the limiting wavelength is only related to the metal material, and has nothing to do with the intensity of the light wave and the irradiation time. The intensity of light quantum depends on the wavelength of light and has nothing to do with the light intensity.

Light Quantum Hypothesis

Einstein boldly assumed that light and atomic electrons are also particle-like. Light is the flow of particles moving at the speed of light C. He calls this light quantum of light. Like Planck's energy, the energy of each light quantum is also E=hν. According to the relativistic equation of mass energy, the momentum of each photon is p=E/c=h/λ.

The formula for the energy of light quantum is: E=hν. Also called the Planck formula. Where h is the Planck constant, h = 6.63 × 10^-34 J·s, and ν is the optical frequency. And hν = 1/2mv2+W (only metal is discussed here), 1/2mv2 is the initial kinetic energy of the photon. When hν=W, the photons just get out of metal. At this time, the limit frequency ν0 = W/h can be calculated from the above formula. When hν

Hν=1/2mv2+W is called Einstein's equation. It is Einstein's new bold assumption based on Planck's theory of quantum: Light is not continuous, but consists of particles one by one. This particle is also called light quanta. And light quantum has a certain energy: E=hν. At the same time, according to the theory of relativity, the momentum of each photon is p=E/c=h/λ. Einstein also used this formula to explain the photoelectric effect. At the same time, he explained the light quantum and obtained the 1921 Nobel Prize in physics.

The light quantum hypothesis successfully explained the photoelectric effect. When ultraviolet rays, such as shorter wavelength light, strike a metal surface, electrons escape from the metal. This phenomenon is called the photoelectric effect. It was discovered by Hertz (HR Hertz l857-1894) and P.Lendard l862-1947. The experiment of the photoelectric effect shows that weak violet light can produce electrons from the metal surface, but strong red light can not produce electrons, that is, the generation of the photoelectric effect depends only on the frequency of light and has nothing to do with the intensity of light. This phenomenon cannot be explained by the fluctuation of light. Because light fluctuations say that light is a kind of wave, its energy is continuous, and it is related to the amplitude of the light wave, that is, intensity, but not to the frequency of the light, that is, the color, if the weak purple light can make electrons from the metal surface, it is very strong. The red light should be better able to produce electrons, but the fact is the opposite. The use of the light quantum hypothesis can satisfactorily explain the photoelectric effect. According to the light quantum hypothesis, light consists of light quantum. The energy of light is discontinuous. The energy of each light quantum must reach a certain value to overcome the electron's work function, and electrons are emitted from the metal surface. Although the number of faint violet light is relatively small, the energy of each light quantum is large enough to produce electrons from the metal surface. The strong red light, although the number of light quantum is large, the energy of each light quantum is not large enough. Overcoming the electron's escaping action can't make electrons.

Hertz confirmed the existence of electromagnetic waves in his own experiments, proclaimed the victory of the light's fluctuations, and sentenced the particles of light to the death penalty, but the photoelectric effect he discovered led to the resurrection of particles.

From the point of view at that time, the light quantum hypothesis contradicted the facts of light and many physicists disapproved of the light quantum hypothesis. Even Planck complained that it was “too much”. In 1907 he wrote to Einstein, he said: : "What I'm looking for as a base photon (light quantum) is not what it means in a vacuum, but what it means in where it is absorbed and emitted, and I think that the vacuum process has been precisely described by Maxwell's equations." . Until 1913 he also rejected the light quantum hypothesis. The American physicist Millikkan (1868–1953) has made outstanding contributions to the study of electronic and photoelectric effects. He spent ten years doing experiments on photoelectric effects. Initially he did not believe in light quantum theory and tried to deny it experimentally, but the result of the experiment was contrary to his original wish. In 1915 he announced that his experiments confirmed the Einstein photoelectric effect formula. He gave the value of h according to the theory of light quantum, which is in good agreement with the value of h given by Planck's radiation formula. Between 1922 and 1923, A. Compton l892-1962 studied the spectrum of X-rays scattered by metals or graphite. According to the classical theory of electromagnetic waves, the incident wavelength should be equal to the scattering wavelength, and Compton's experiments have found that in addition to wavelength-invariant scattering, there are scatterings larger than the incident wavelength. This wavelength-changing scattering is known as CommScope. Effect. The fluctuation of light cannot explain this effect anyway, and the light quantum hypothesis can successfully explain it. According to the theory of light quantum, the incident X-ray is a photon beam. When the photon collides with the free electrons in the scatterer, it will give a part of its own energy to the electrons. Since the scattered photon energy is reduced, the photon frequency is reduced, and the wavelength Bigger. Therefore, the discovery of the Compton effect strongly confirmed the light quantum theory.

Einstein's light quantum theory developed the quantum theory pioneered by Planck. In Planck's theory, it is still insisted that the electromagnetic waves are continuous in nature, but it is only assumed that the electromagnetic energy shows quantum when they exchange energy with the wall oscillator. Einstein did not adopt an improved attitude toward the old theory, but asked to clarify the essence of things to completely solve the problem. He saw that quantum is not a successful mathematical formula, but a means to expose the essence of light. He overcame the incompleteness of the Planck Quantum Hypothesis, extending the quantum from the mechanism of radiation to the light itself, arguing that the light itself is also discontinuous. Light is not only quantized during absorption and emission, but also light. The spread itself is also quantified. Einstein’s light quantum hypothesis restores the particle nature of light, enabling people to finally recognize the wave-particle dual nature of light. Inspired by it, he discovered the de Broglie material wave and made people recognize the waves of the microscopic world. Particle duality lays the foundation for the later establishment of quantum mechanics.

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