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

Quando a luz de um determinado comprimento de onda e frequência atinge um metal, os elétrons são expulsos do metal. Este fenómeno é conhecido como o efeito fotoelétrico. No entanto, apenas a luz acima de uma frequência limite pode expulsar elétrons a partir do metal.A luz de baixa frequência não expulsa elétrons, independentemente da sua intensidade. Como é que isto acontece, Albert Einstein propôs que a luz se comporta como um fluxo de partículas ou pequenos pacotes. Um pacote ou quantum de luz é chamado de fóton.Cada fóton tem uma energia, E, que depende da sua frequência, nu. As duas estão relacionadas por esta equação, onde h é a constante de Planck e tem um valor de 6, 626 vezes 10 para os 34 joule-segundos negativos. A luz, e, portanto, os fótons, de maior frequência têm maior energia.Um elétron está ligado a um metal com uma energia de ligação, phi. Isto também é conhecido como a função de trabalho do metal, W.Portanto, é necessária uma quantidade de energia superior a W superior a W para superar as forças atrativas e desalojar o elétron. Quando um fóton com energia superior a W atinge o metal, o seu excesso de energia é transferido para o elétron como energia cinética e o elétron é expulso.Para resumir o efeito fotoelétrico, a luz de baixa frequência, luz de longo comprimento de onda não expulsa quaisquer elétrons do metal. Aumentar a intensidade da luz de baixa frequência apenas aumenta o número de fótons. Cada fóton ainda tem a mesma energia, o que não é suficiente para desalojar quaisquer elétrons.Luz de comprimento de onda mais curto com uma frequência mais alta do que a frequência limite expulsa os elétrons porque cada fóton tem maior energia. À medida que a frequência da luz aumenta, o mesmo acontece com a energia do fóton, e, portanto, a energia cinética do elétron ejetado. Quanto maior for a intensidade da luz, quanto mais fótons atingirem o metal, maior o número de elétrons que são expulsos.O efeito fotoelétrico demonstra o comportamento das partículas de luz.

efeito fotoelétrico

Quando luz de um comprimento de onda específico atinge uma superfície metálica, são emitidos eletrões. Isto é chamado de efeito fotoelétrico. A frequência mínima de luz que pode causar essa emissão de eletrões é denominada frequência limite, que é específica do metal. A luz com uma frequência inferior à frequência limite, mesmo que seja de alta intensidade, não pode iniciar a emissão de eletrões. No entanto, quando a frequência é superior ao valor limite, o número de eletrões ejetados é diretamente proporcional à intensidade do feixe.

De acordo com a teoria clássica da onda, a energia de uma onda depende da sua intensidade (que depende da sua amplitude), e não da sua frequência. Uma parte destas observações foi que se viu que o número de eletrões ejetados em um determinado período de tempo aumentava à medida que o brilho aumentava. Em 1905, Albert Einstein foi capaz de resolver o paradoxo ao incorporar as descobertas de quantização de Planck na visão particular desacreditada da luz.

Einstein argumentou que as energias quantizadas que Planck tinha postulado poderiam ser aplicadas à luz no efeito fotoelétrico. A luz que embate na superfície metálica não deve ser vista como uma onda, mas deve ser vista em vez disso como um fluxo de partículas (mais tarde chamadas fotões) cuja energia depende da sua frequência. A quantidade de energia (E) em um pacote luminoso depende da sua frequência (ν), de acordo com a seguinte equação:

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/976/976_Equations-01.png" style="height: 40px;" />

onde h é a constante de Planck.

O efeito fotoelétrico pode ser descrito assumindo que a luz é quantizada. É necessária uma determinada energia mínima para ultrapassar a energia de ligação (Φ) experienciada por um eletrão. Isto também é conhecido como a função de trabalho (W) do metal.

Como os eletrões no metal têm uma certa quantidade de energia de ligação que os mantém lá, a luz incidente precisa de ter mais energia para libertar os eletrões. Os fotões de luz de baixa frequência não contêm energia suficiente para ejetar eletrões do metal. Mesmo que o metal seja exposto a tal luz por um longo período de tempo, não é observada emissão de eletrões. Um eletrão só pode ser emitido quando um fotão com energia superior à função de trabalho atinge o metal.

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/976/976_Equations-02.png" style="height: 40px;" />

A energia em excesso do fotão é convertida em energia cinética do eletrão emitido.

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/976/976_Equations-03.png" style="height: 40px;" />

Portanto, os eletrões são ejetados quando são atingidos por fotões com energia suficiente (uma frequência superior ao limite). Quanto maior a frequência da luz incidente, maior a energia cinética transmitida pelas colisões para os eletrões que escapam. Einstein também argumentou que a intensidade da luz não dependia da amplitude da onda de entrada, mas sim do número de fotões que batem na superfície dentro de um determinado período de tempo. O número de eletrões ejetados aumenta com o brilho. Quanto maior for o número de fotões de entrada, maior será a probabilidade de colidirem com alguns dos eletrões.

O efeito fotoelétrico sugere fortemente o comportamento particular da luz. Einstein ganhou o Prémio Nobel de Física em 1921 pela sua explicação do efeito fotoelétrico. Embora muitos fenómenos de luz possam ser explicados tanto em termos de ondas como de partículas, certos fenómenos, como os padrões de interferência obtidos quando a luz passa por uma fenda dupla, eram completamente contrários a uma visão particular da luz, enquanto que outros fenómenos, como o efeito fotoeléctrico, eram completamente contrários a uma visão ondulatória da luz. De algum modo, a um nível fundamental profundo ainda não totalmente compreendido, a luz é ao mesmo tempo ondulatória e particular. Isto é conhecido como dualidade onda-partícula.

Este texto foi adaptado de <a href="https://openstax.org/books/chemistry-2e/pages/6-1-electromagnetic-energy">Openstax, Chemistry 2e, Section 6.1: Electromagnetic Energy</a>.

Efeito Fotoelétrico

Quando a luz de um determinado comprimento de onda atinge uma superfície metálica, os elétrons são emitidos. Isso é chamado de efeito fotoelétrico. A frequência mínima de ...

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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...

the electromagnetic spectrum

Electromagnetic Waves: Uses Across the Spectrum

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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

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emission spectra

Line Spectra from Heated Gases

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the de broglie wavelength

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the uncertainty principle

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the quantum-mechanical model of an atom

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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

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atomic orbitals

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the pauli exclusion principle

Electron Spin and Orbital Pairing

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the energies of atomic orbitals

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the aufbau principle and hund's rule

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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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Motion And Stability: Forces And Interactions

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Why Light Ejects Electrons from Metals

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