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

Когда свет с определенной длиной волны и частоты падает на металл, он выбивает электроны из металла. Это явление известно как фотоэлектрический эффект. Однако только свет с частотой выше порогового значения может выбивать электроны из металла.Свет низкой частоты не выбивает электроны, независимо от его интенсивности. Почему так происходит? Альберт Эйнштейн предположил, что свет ведет себя как поток частиц или крошечных пучков.Один пучок или квант света называется фотоном. Каждый фотон имеет энергию E, которая зависит от его частоты nu. Они связаны следующим уравнением, где h постоянная Планка и имеет значение 6, 626 умноженное на 10 в минус 34 степени джоулей в секунду.Свет и, следовательно, фотоны более высокой частоты обладают большей энергией. Электрон связан с металлом с энергией связи фи. Она также известна как работа выхода металла W.Таким образом, для преодоления сил притяжения и выбивания электрона требуется энергия, превышающая W.Когда фотон с энергией выше W ударяется о металл, его избыточная энергия передается электрону в виде кинетической энергии, и электрон покидает атом.Подводя итог явления фотоэлектрического эффекта, можно сделать вывод, что низкочастотный длинноволновый свет не выбивает электроны из металла. Увеличение интенсивности низкочастотного света только увеличивает количество фотонов. Каждый фотон по-прежнему имеет одинаковую энергию, которой недостаточно, чтобы выбить электроны.Свет с меньшей длиной волны и частотой выше пороговой действительно выбивает электроны, потому что каждый фотон имеет более высокую энергию. По мере увеличения частоты света увеличивается энергия фотона и, следовательно, кинетическая энергия выбитого электрона. Чем больше интенсивность света, тем больше фотонов ударяется о металл и тем больше электронов выбивается.Фотоэлектрический эффект демонстрирует поведение частиц света.

Фотоэлектрический эффект

Когда свет определенной длины волны попадает на металлическую поверхность, электроны испускаются. Это называется фотоэффектом. Минимальная частота света, которая может вызвать такое излучение электронов, называется пороговой частотой, которая характерна для металла. Свет с частотой ниже пороговой частоты, даже если он имеет высокую интенсивность, не может инициировать излучение электронов. Однако если частота превышает пороговое значение, количество выбрасываемого электронов прямо пропорционально интенсивности пучка. 
 
 Согласно классической волновой теории, энергия волны зависит от ее интенсивности (которая зависит от ее амплитуды), а не от ее частоты. Одна из частей этих наблюдений заключалась в том, что по мере увеличения яркости количество электронов, выброшенных в течение заданного периода времени, увеличивалось. В 1905 году Альберт Эйнштейн смог разрешить этот парадокс, включив результаты квантования Планка в дискредитированное представление о частице света. 
 
 Эйнштейн утверждал, что квантованные энергии, которые постулировал Планк, могут быть применены к свету в фотоэффекте. Свет, ударяя по металлической поверхности, не следует рассматривать как волну, Но вместо этого следует рассматривать как поток частиц (впоследствии называемых фотонами), энергия которых зависела от их частоты, количество энергии (E) в пакете света зависит от его частоты (ν) по следующей формуле: 
 
 <img alt=""Eq1"" src=""https://www.jove.com/files/core/CDNSource/lessons/976/976_Equations-01.png"" style=""height: 40px;"" />
 
 Где h – константа Планка. 
 
 Фотоэлектрический эффект можно описать, предположив, что свет квантизирован. Для преодоления энергии связывания (Φ), испытываемой электроном, требуется определенная минимальная энергия. Это также называется рабочей функцией (W) металла. 
 
 Так как электроны в металле имеют определенное количество энергии связывания, чтобы удержать их там, падающий свет должен иметь больше энергии, чтобы освободить электроны. Фотоны низкочастотного света не содержат достаточного количества энергии для выброса электронов из металла. Даже если металл длительное время подвергается воздействию такого света, излучение электронов не наблюдается. Электрон может быть испущен только в том случае, если фотон с энергией, превышающей рабочую функцию, ударяет по металлу. 
 
 <img alt=""Eq1"" src=""https://www.jove.com/files/core/CDNSource/lessons/976/976_Equations-02.png"" style=""height: 40px;"" />
 
 Избыточная энергия фотона преобразуется в кинетическую энергию испускаемого электрона. 
 
 <img alt=""Eq1"" src=""https://www.jove.com/files/core/CDNSource/lessons/976/976_Equations-03.png"" style=""height: 40px;"" />
 
 Таким образом, электроны выбрасываются при попадании фотонов с достаточной энергией (частота превышает пороговое значение). Чем больше частота падающего света, тем больше кинетическая энергия, передаваемая столкновениями с выделяемыми электронами. Эйнштейн также утверждал, что интенсивность света не зависела от амплитуды входящей волны, а вместо этого соответствовала количеству фотонов, поражающих поверхность в течение заданного периода времени. Количество выбрасанных электронов увеличивается с увеличением яркости. Чем больше количество входящих фотонов, тем выше вероятность столкновения с некоторыми электронами. 
 
 Фотоэлектрический эффект убедительно указывает на поведение частиц света. Эйнштейн получил Нобелевскую премию по физике в 1921 году за объяснение фотоэлектрического эффекта. Хотя многие световые явления можно объяснить либо с помощью волн, либо с помощью частиц, некоторые явления, такие как интерференционные картины, полученные при прохождении света через двойную щель, полностью противоречили представлению о свете частицами, в то время как другие явления, такие как фотоэлектрические эффект, были полностью противоположны волновому взгляду на свет. Каким-то образом на глубоком фундаментальном уровне, все еще не полностью изученном, свет одновременно волнообразен и подобен частицам. Это известно как дуальность волна-частица. 
 
 Этот текст адаптирован из <a href=""https://openstax.org/books/chemistry-2e/pages/6-1-electromagnetic-energy"">Openstax, Химия 2е изд., раздел 6.1: Электромагнитная энергия</a>.

Когда свет определенной длины волны попадает на металлическую поверхность, электроны испускаются. Это называется фотоэффектом. Минимальная частота ...

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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...

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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,...

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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...

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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...

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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...

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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...

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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

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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

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Why Light Ejects Electrons from Metals

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