Name: why light ejects electrons from metals
Uploaded: 2020-09-03T13:38:27.000Z
Duration: 2 min 26 s
Channel: JoVE

K12 Standards

Pennsylvania State Academic Standards

9-12

Physical Science

Waves and Electromagnetic Radiation

3.2.9-12.X

Photoelectric Effect

当特定波长和频率的光 照射金属时，电子就会从金属中喷射出来。这种现象被称为光电效应。然而，只有高于阈值频率的光 才能从金属中喷射出电子。较低频率的光不喷射电子，不管其强度如何。这是怎么回事呢？阿尔伯特·爱因斯坦提出光 表现为一束粒子流或一个小包。光的一个包—或量子—称为光子。每个光子都有一个能量，E，这个能量 取决于它的频率 nu（ν）二者由这个方程式联系在一起，其中 h 是普朗克常数，它的值为 6.626 乘以 10 的 负 34 次方焦耳秒。高频率的光和光子 具有更大的能量。电子以结合能 φ 与金属结合。这也被称为金属的功函数，W。所以，需要一个大于 W 的能量 来克服这个吸引力 并使电子移位。当一个能量大于 W 的光子 撞击金属时，其多余的能量 作为动能传递给电子，电子被喷射出去。总结一下光电效应，低频率长波长的光 不会从金属中喷射出任何电子。增加低频光的强度 只会增加光子的数量。每个光子仍然具有相同的能量，这个能量不足以移动任何电子。频率高于阈值频率的短波长光 确实会发射电子，因为每个光子都是更高的能量。当光的频率增加时，光子的能量也会增加，因此，喷射出的电子的动能也会增加。光的强度越大，撞击金属的光子就越多，喷射出的电子数量也就越多。光电效应证明了 光的粒子行为。

光电效应

当特定波长的光入射到金属表面时，会发射出电子。这称为光电效应。可以引起电子发射的最小光频率称为阈值频率，该频率特定于金属。频率低于阈值频率的光，即使强度很高，也无法启动电子发射。但是，当频率高于阈值时，射出的电子数量与电子束的强度成正比。
 
 根据经典波理论，波的能量取决于其强度（取决于其幅度），而不是其频率。这些观察结果的一部分是，在给定的时间段内，随着亮度的增加，射出的电子数量会增加。 1905年，爱因斯坦（Albert Einstein）通过将普朗克（Planck）的量化结果整合到信誉良好的光粒子视图中，解决了这一悖论。
 
 爱因斯坦认为，普朗克假定的量子化能量可以通过光电效应应用于光。撞击金属表面的光不应视为波，而应视为其能量取决于其频率的粒子流（以后称为 光子），能量的量（<em光包中的> E ）取决于其频率（＆nu;），根据以下公式：
 
 <img alt =“ Eq1” src =“ https://www.jove.com/files/core/CDNSource/lessons/976/976_Equations-01.png” style =“ height：40px ;“ /> 
 
 其中 h 是普朗克常数。
 
 可以通过假设光被量化来描述光电效应。需要一定的最小能量来克服电子所经历的结合能（Φ）。这也称为金属的功函数（ W ）。
 
 由于金属中的电子具有一定数量的结合能，使它们保持在那里，因此入射光需要具有更多的能量来释放电子。低频光子的能量不足，无法从金属中喷射出电子。即使金属长时间暴露在这种光下，也没有观察到电子的发射。仅当能量大于功函数的光子撞击金属时，电子才能发射。
 
 <img alt =“ Eq1” src =“ https://www.jove.com/files/core/CDNSource/lessons/976/976_Equations-02.png” style =“height: 40px ;“ /> 
 
 光子的多余能量转换为发射电子的动能。
 
 <img alt =“ Eq1” src =“ https://www.jove.com/files/core/CDNSource/lessons/976/976_Equations-03.png” style =“ height：40px ;“ /> 
 
 因此，电子在被具有足够能量（频率大于阈值）的光子撞击时会被弹出。入射光的频率越高，碰撞传递给逃逸电子的动能就越大。爱因斯坦还认为，光强度不取决于入射波的幅度，而是与给定时间段内撞击表面的光子数量相对应。射出的电子数量随亮度增加而增加。入射光子的数量越多，它们与某些电子发生碰撞的可能性就越大。
 
 光电效应强烈暗示了光的粒子行为。爱因斯坦因对光电效应的解释而赢得了1921年诺贝尔物理学奖。尽管可以用波或粒子来解释许多光现象，但是某些现象（例如当光通过双缝时获得的干涉图样）与光的粒子视图完全相反，而其他现象（例如光电）效果，完全违背了光波的观点。不知何故，在一个尚未被完全理解的深层基础上，光既是波状的又是粒子状的。这就是所谓的波粒对偶。
 
 本文改编自<a href="https://openstax.org/books/chemistry-2e/pages/6-1-electromagnetic-energy"> Openstax，化学2e，第6.1节：电磁能量</a>。

当特定波长的光照射到金属表面时，会发射出电子，这一现象称为光电效应。其最小频率……

Education

教育

JoVE Core

Chemistry

化学

Electronic Structure of Atoms

原子的电子结构

KEY TERMS AND CONCEPTS

关键术语与概念

why light ejects electrons from metals

Light can eject electrons from a metal surface when it has the right frequency. This process is called the photoelectric effect. Each metal has its own threshold frequency, which is the minimum light frequency needed to free electrons.

Light below the threshold frequency cannot cause emission, even if it is very bright. When the light frequency is above the threshold, electrons are ejected. In that case, the number of electrons released is directly proportional to the intensity, or brightness, of the beam.

Classical wave theory could not explain this result because it links a wave’s energy to its intensity, not its frequency. In 1905, Albert Einstein solved the problem by using Planck’s idea that energy is quantized. He treated light as a stream of particles called photons, and each photon carries energy based on its frequency.

A metal holds onto its electrons with a minimum amount of binding energy. This energy barrier is called the work function, or Φ, of the metal. Low-frequency photons do not have enough energy to overcome it, so no electrons are emitted even after long exposure. If a photon has more energy than the work function, the extra energy becomes kinetic energy of the escaping electron.

Higher-frequency light gives electrons more kinetic energy. Brightness also matters because intensity reflects the number of photons striking the surface in a given time. More photons increase the chance that some will hit electrons and free them.

The photoelectric effect supports the particle behavior of light. Einstein won the Nobel Prize in Physics in 1921 for explaining it. At the same time, other light behavior, such as the interference pattern in a double slit, does not fit a simple particle model. Together, these observations show wave-particle duality, the idea that light has both wavelike and particle-like properties.

Why Light Ejects Electrons from Metals

the wave nature of light

Measuring Light with Wavelength and Frequency

Light can be described as an electromagnetic wave, and its behavior helps explain many patterns seen in science. For centuries, scientists have tried to understand light. Newton used lenses and prisms to show that white light is made of the colors of the rainbow mixed together. He explained light as tiny particles moving at high speed. Other scientists, including Christiaan Huygens, showed that reflection and refraction could also be explained by treating light as a wave. At the time, people...

the electromagnetic spectrum

Electromagnetic Waves: Uses Across the Spectrum

The electromagnetic spectrum includes all electromagnetic radiation arranged by frequency and wavelength. Visible light is only a small part of it, and each color of visible light has its own frequency and wavelength. Different parts of the spectrum also use different units. Radio waves are usually measured in MHz, while the visible range is often measured in nm or angstroms. The spectrum is organized from radio waves to gamma rays as frequency increases and wavelength decreases. Radio waves...

interference and diffraction

Wave Patterns from Light and Slits

Light can form wave patterns through interference and diffraction. When two electromagnetic waves meet with their peaks and troughs lined up, they combine into a larger wave. This is constructive interference, and the waves are in phase. When a peak of one wave meets a trough of the other, the waves reduce each other. This is destructive interference, and the result has a much lower amplitude. Diffraction is another wave behavior. When light passes through a slit that is about the same size as...

photoelectric effect

Light can eject electrons from a metal surface when it has the right frequency. This process is called the photoelectric effect. Each metal has its own threshold frequency, which is the minimum light frequency needed to free electrons. Light below the threshold frequency cannot cause emission, even if it is very bright. When the light frequency is above the threshold, electrons are ejected. In that case, the number of electrons released is directly proportional to the intensity, or brightness,...

the bohr model

Bohr’s Quantized Atomic Orbits

Bohr’s quantized atomic orbits explain why hydrogen atoms can stay stable. Early in the twentieth century, Ernest Rutherford and his colleagues described atoms as tiny dense nuclei with lighter electrons moving around them. This picture was called the planetary model because it compared the atom to a miniature solar system, with electrons orbiting the nucleus like planets around the sun. The simplest atom is hydrogen. It has one proton in the nucleus and one electron moving around it. In the...

emission spectra

Line Spectra from Heated Gases

Line spectra are produced when heated gases give off light at specific wavelengths. In contrast, hot solids, liquids, or condensed gases radiate excess energy as a continuous spectrum. That spectrum contains an unbroken range of wavelengths. A gas at low partial pressure can produce a line spectrum when it is excited by an electric current or heated. Fluorescent light bulbs and neon signs work in this way. Each element has its own characteristic set of lines, and molecules can also produce...

the de broglie wavelength

Electron Waves and Standing Orbits

Electron waves and standing orbits explain how tiny particles can behave like waves. Large objects in the macroscopic world, like a billiard ball on a table, follow classical physics. A ball keeps moving in a straight line unless it hits another ball or a force such as friction acts on it. It also has a well-defined position, velocity, and momentum, where momentum is mass times velocity. Waves behave differently from macroscopic particles. When waves interact, they can form interference...

the uncertainty principle

Electron Position and Momentum Limits

Heisenberg’s uncertainty principle sets a limit on how precisely we can know an electron’s position and momentum at the same time. Momentum is the motion of a particle, and the two measurements affect each other. If the momentum is known more accurately, the position becomes less certain, and the reverse is also true. Heisenberg described this limit with an equation that uses Planck’s constant. The equation shows the smallest possible uncertainty for position and momentum together. It explains...

the quantum-mechanical model of an atom

Electron Orbitals in Quantum Mechanics

Quantum mechanics explains how electrons behave in atoms using waves and probability. Erwin Schrödinger built on de Broglie’s idea that an electron in hydrogen could act like a circular standing wave instead of moving in fixed circular paths. He then derived the Schrödinger equation, which became a major step in modern atomic theory. When Schrödinger applied his equation to hydrogen-like atoms, it matched Bohr’s energy expression and the Rydberg formula for hydrogen spectra. He described...

quantum numbers

Electron Orbitals and Spin

Electron orbitals are described by quantum numbers that show where an electron is most likely to be found around the nucleus. These numbers come from the quantum mechanical model, which is based on solutions of the Schrödinger equation. They also help explain why electron energy comes in set values instead of changing smoothly. The principal quantum number, n, labels the energy level. It can be 1, 2, 3, and so on, and higher n values usually mean higher electron energy. This number also tells...

atomic orbitals

Electron Regions in Atomic Orbitals

Electron regions in atomic orbitals show where an electron is most likely to be found in an atom. These three-dimensional regions are often drawn as boundary surfaces around the densest part of the cloud. The shape of an orbital depends on the angular momentum quantum number, l. This value can range from 0 to n - 1, where n is the principal quantum number. For n = 1, only l = 0 is allowed. For n = 2, both l = 0 and l = 1 are allowed. Orbitals with the same value of l form a subshell. The l =...

the pauli exclusion principle

Electron Spin and Orbital Pairing

Electron spin and orbital pairing explain why only two electrons can fit in one atomic orbital. Electron configuration describes how electrons are arranged in an atom’s orbitals. It is written with three parts: the principal quantum number, n, the orbital type, or subshell, and a superscript that shows how many electrons are in that subshell. For example, 2p4 means four electrons in a p subshell with n = 2, and 3d8 means eight electrons in a d subshell with n = 3. The three quantum numbers...

the energies of atomic orbitals

Shielding and Orbital Penetration

Shielding and orbital penetration help explain why electrons in an atom have different energies. Electrons are attracted to the positive nucleus, but they also repel one another in atoms with more than one electron. The strength of these forces depends on the charge and the distance between the particles. Opposite charges attract, like charges repel, and the force gets weaker as the particles move farther apart. An electron that stays closer to the nucleus is held more tightly. That means...

the aufbau principle and hund's rule

Electron Filling Order in Atomic Orbitals

Electron filling order in atomic orbitals follows the aufbau principle and Hund's rule. To write an electron configuration, we build atoms in order of atomic number. Starting with hydrogen and moving across the periods of the periodic table, one proton is added to the nucleus and one electron is placed into the correct subshell. This process describes the ground-state electron configurations of the elements. The aufbau principle comes from the German word aufbau, which means "to build up." It...

electron configuration of multielectron atoms

electron-configuration-of-multielectron-atoms

Valence Electrons and Noble Gas Shorthand

Valence electrons and noble gas shorthand help show how electrons are arranged in multielectron atoms. Sodium, the alkali metal with atomic number 11, has one more electron than neon. That extra electron goes into the lowest-energy subshell available, the 3s orbital, so sodium has the configuration 1s2 2s2 2p6 3s1. Electrons in the outermost shell, where n is highest, are called valence electrons. Electrons in the inner shells are called core electrons. Because core electrons often match a...

key terms and concepts

Explore how light frees electrons from metals, why threshold frequency matters, and how Einstein’s photon idea explained the photoelectric effect.

Why Light Ejects Electrons from Metals | JoVE K12

chemistry

JoVE Core 化学通过简单明了且通俗易懂的动画视频课程，将普通化学概念带入到现实生活当中。此外，在科学家行动视频中，我们演示了在实验室中进行的经典和原始研究实验。

Louisiana Student Standards for Science

High School

motion-and-stability-forces-and-interactions

Motion And Stability: Forces And Interactions

HS-PS2-6

ohios-learning-standards-and-model-curriculum

Ohio’s Learning Standards and Model Curriculum

Physics

Waves

Light phenomena

Tennessee Academic Standards

waves-and-their-applications-in-technologies-for-information-transfer

Waves and Their Applications in Technologies for Information Transfer

PHYS.PS4.7

CHEM1.PS4.1

Arizona Science Standards

HS+C.P1

HS+C.P1U1.2

Next Generation Science Standards

HS-PS4-4

Florida Department of Education

SC.912.P.8

SC.912.P.8.3

Texas Essential Knowledge and Skills

PHYS.9

PHYS.9.A

south-carolina-college-and-career-ready-science-standards

South Carolina College and Career Ready Science Standards

C-PS4-5

new-york-state-science-learning-standards

New York State Science Learning Standards

HS. Chemical Reactions

HS-PS1-7.

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Why Light Ejects Electrons from Metals

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