定义Definition
光电效应是光照射金属等物质表面时，频率足够高的光使电子从表面逸出的现象。逸出电子的最大动能只与光的频率有关、与光强无关，存在一个截止频率，这些规律只能用光的量子理论解释。
The photoelectric effect is the emission of electrons from a metal or other surface when it is illuminated by light of high enough frequency. The maximum kinetic energy of the emitted electrons depends only on the frequency, not the intensity, and there is a threshold frequency; these facts can be explained only by the quantum theory of light.
原理Principle
爱因斯坦光电方程 E_k,max = hν − W：一个光子把全部能量 hν 交给一个电子，其中一部分用于克服金属的逸出功 W，余下成为电子动能。频率低于 ν₀ = W/h 时，无论光多强都没有电子逸出；光强只决定逸出电子的数目。这与经典波动理论（能量随光强连续积累）完全矛盾。铯的逸出功约 2.1 eV，可见光就能使其发射电子。
Einstein's equation E_k,max = hν − W says one photon gives all its energy hν to one electron; part overcomes the metal's work function W and the rest becomes kinetic energy. Below ν₀ = W/h no electrons emerge however bright the light, and intensity sets only the number of electrons. This contradicts classical wave theory, in which energy would accumulate with intensity. Caesium's work function is about 2.1 eV, so visible light ejects its electrons.
发现与解释Discovery and explanation
1887 年赫兹在验证电磁波时发现紫外光照射使电火花更容易产生。1902 年勒纳德系统研究发现电子动能与光强无关。1905 年爱因斯坦提出光量子假说解释光电效应。1916 年密立根用精密实验验证了爱因斯坦方程并测得普朗克常数。爱因斯坦因光电效应定律获 1921 年诺贝尔物理学奖。
In 1887 Hertz, testing electromagnetic waves, noticed that ultraviolet light made sparks jump more easily. In 1902 Lenard found that electron energy does not depend on light intensity. In 1905 Einstein explained the effect with his light-quantum hypothesis, and in 1916 Millikan's precise experiments confirmed Einstein's equation and measured Planck's constant. Einstein received the 1921 Nobel Prize in Physics for the law of the photoelectric effect.
应用Applications
光电效应用于光电管、光电倍增管、夜视仪和图像传感器。化学中的光电子能谱（紫外光电子能谱 UPS 和 X 射线光电子能谱 XPS）就是基于光电效应，测量电子结合能以研究分子轨道能级和表面元素组成。太阳能电池利用的是内光电效应（光生伏打效应）。
The effect is used in phototubes, photomultipliers, night-vision devices and image sensors. Photoelectron spectroscopy in chemistry, UPS and XPS, is based on it, measuring electron binding energies to study orbital energies and surface composition. Solar cells exploit the related internal photoelectric (photovoltaic) effect.
反应Reactions
eU₀ = E_k,max，U₀ 为遏止电压。
eU₀ = E_k,max, where U₀ is the stopping voltage.
计算示例
A worked example
年表Timeline
- 1887
赫兹发现光电效应
Hertz discovers the photoelectric effect
- 1902
勒纳德的系统研究
Lenard's systematic study
- 1905
爱因斯坦光量子解释
Einstein's light-quantum explanation
- 1916
密立根实验验证
Millikan's experimental confirmation
- 1921
爱因斯坦获诺贝尔物理学奖
Einstein receives the Nobel Prize in Physics
相关公式Formulas
例题Worked example
钠 Φ = 2.28 eV（ν₀ = 5.5×10¹⁴ Hz，λ₀ = 544 nm）；用 400 nm 光照射时 E_k,max = 3.10 − 2.28 = 0.82 eV。
Sodium Φ = 2.28 eV; 400 nm light gives E_k,max = 0.82 eV.