Wave-Particle Duality

Publicized early in the debate about whether light was composed of particles or waves, a wave-particle dual nature soon was found to be characteristic of electrons as well. The evidence for the description of light as waves was well established at the turn of the century when the photoelectric effect introduced firm evidence of a particle nature as well. On the other hand, the particle properties of electrons was well documented when the DeBroglie hypothesis and the subsequent experiments by Davisson and Germer established the wave nature of the electron.

The Photoelectric Effect

The details of the photoelectric effect were in direct contradiction to the expectations of very well developed classical physics. 光电效应的细节与高度发展的经典物理学的预期直接矛盾。 The explanation marked one of the major steps toward quantum theory. 该解释是量子理论发展中的一个关键步骤

The remarkable aspects of the photoelectric effect when it was first observed were: 当光电效应首次被观察到时，其显著特点包括： 1. The electrons were emitted immediately - no time lag! 电子立即被发射出来——没有时间延迟！ 2. Increasing the intensity of the light increased the number of photoelectrons, but not their maximum kinetic energy! 增加光强使光电子数量增加，但未改变它们的最大动能！ 3. Red light will not cause the ejection of electrons, no matter what the intensity! 红光不会导致电子逸出，无论光强如何！ 4. A weak violet light will eject only a few electrons, but their maximum kinetic energies are greater than those for intense light of longer wavelengths! 一种微弱的紫光只能使少数电子发生光电效应，但这些电子的最大动能大于强光中较长波长光的电子最大动能！

Photoelectric Effect

Experiment 实验

Analysis of data from the photoelectric experiment showed that the energy of the ejected electrons was proportional to the frequency of the illuminating light. This showed that whatever was knocking the electrons out had an energy proportional to light frequency. The remarkable fact that the ejection energy was independent of the total energy of illumination showed that the interaction must be like that of a particle which gave all of its energy to the electron! This fit in well with Planck's hypothesis that light in the blackbody radiation experiment could exist only in discrete bundles with energy 光电实验的数据分析表明，被照射出的电子的能与照明光的频率成正比。这表明 whatever 是敲出电子的东西其能量与光的频率成正比。电子被击出的能量与总照明能量无关这一事实非常显著，这表明相互作用必须像一个粒子那样，将全部能量传递给电子！这与普朗克关于黑体辐射实验中光只能以离散的包络形式存在的假设相吻合。 E = hν Illustration and example 示例和说明

Wave-Particle Duality: Light

Does light consist of particles or waves? When one focuses upon the different types of phenomena observed with light, a strong case can be built for a wave picture:

Interference 干涉

Diffraction 衍射

Polarization 极化

By the turn of the 20th century, most physicists were convinced by phenomena like the above that light could be fully described by a wave, with no necessity for invoking a particle nature. But the story was not over.

Phenomenon 现象	Can be explained in terms of waves. 可以用水波来解释。	Can be explained in terms of particles. 可以解释为粒子的特性。
Reflection 反射
Refraction 折射
Interference 干涉
Diffraction 衍射
Polarization 极化
Photoelectric effect 光电效应

Most commonly observed phenomena with light can be explained by waves. But the photoelectric effect suggested a particle nature for light. Then electrons too were found to exhibit dual natures.

There is a considerable amount of discussion in the literature about whether reflection and refraction of light at an interface where the medium changes can be described in particle terms. For practical applications of geometrical optics, treating light as a wave phenomenon is adequate. 文献中关于在介质变化的界面处，光的反射和折射是否能用粒子观点来描述存在大量讨论。对于几何光学的实用应用，将光视为波现象是足够的。

Newton's corpuscular theory: Newton proposed that light was composed of corpuscles that traveled in straight lines. That worked fine for reflection, because the bouncing of either particles or waves off a planar surface follows the same law of reflection. But to explain refraction, he had to presume that the particles traveled faster in a more optically dense material. But Foucault's 1850 experiment showed that light traveled more slowly in such media, so that version of a particle theory of light had to be set aside.

Huygens wave theory: Presuming that light was composed of waves, in 1678 Huygens proposed that each point of a light wavefront could be considered to be the source of a spherical wave. The Huygens' principle helped develop the wave theory of light and it was further developed by Fresnel and Kirchhoff.

Quantum view of light: The photoelectric effect introduced evidence that light exhibited particle properties on the quantum scale of atoms. At least light can achieve a sufficient localization of energy to eject an electron from a metal surface. So it might be implied that there must be a particle treatment of refraction of light, but for ordinary optics the wave view of light is the practical approach.

Photoelectric Effect

Most commonly observed phenomena with light can be explained by waves. But the photoelectric effect suggested a particle nature for light.