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

Cuando la luz con una determinada longitud de onda y frecuencia golpea un metal, los electrones son expulsados del metal. Este fenómeno se conoce como efecto fotoeléctrico. Sin embargo, solo la luz por encima de un umbral de frecuencia puede expulsar electrones del metal.La luz de baja frecuencia no expulsa electrones, independientemente de su intensidad. Cómo ocurre esto? Albert Einstein propuso que la luz se comporta como una corriente de partículas o pequeños paquetes.Un paquete, o quantum, de luz se llama fotón. Cada fotón tiene una energía, E, que depende de su frecuencia, nu. Las dos están relacionadas por esta ecuación, donde h es la constante de Planck y tiene un valor de 6.626 veces 10 a los 34 julios-segundos negativos.La luz, y por lo tanto los fotones, de mayor frecuencia tienen una mayor energía. Un electrón está unido a un metal con una energía de enlace, phi. Esto también se conoce como la función de trabajo del metal, W.Entonces, una cantidad de energía mayor que W se requiere para superar las fuerzas de atracción y desalojar el electrón.Cuando un fotón con energía superior a W golpea el metal, su exceso de energía se transfiere al electrón como energía cinética y el electrón es expulsado. Para resumir el efecto fotoeléctrico, la luz de baja frecuencia y longitud de onda larga no expulsa electrones del metal. Aumentar la intensidad de la luz de baja frecuencia solo aumenta el número de fotones.Cada fotón todavía tiene la misma energía, lo cual no es suficiente para desalojar algún electrón. La luz de la longitud de onda más corta con una frecuencia más alta que el umbral de frecuencia expulsa los electrones porque cada fotón es de mayor energía. A medida que aumenta la frecuencia de la luz, también lo hace la energía del fotón, y así, la energía cinética del electrón expulsado.Cuanto mayor sea la intensidad de la luz, más fotones golpean el metal, y mayor será el número de electrones que se expulsan. El efecto fotoeléctrico demuestra el comportamiento de las partículas de la luz.

el efecto fotoeléctrico

Cuando la luz de una longitud de onda particular golpea una superficie metálica, se emiten electrones. Esto se denomina efecto fotoeléctrico. La frecuencia mínima de luz que puede causar tal emisión de electrones se llama frecuencia umbral, que es específica para el metal. La luz con una frecuencia inferior a la frecuencia umbral, incluso si es de alta intensidad, no puede iniciar la emisión de electrones. Sin embargo, cuando la frecuencia es superior al valor umbral, el número de electrones emitidos es directamente proporcional a la intensidad del haz.
 
 Según la teoría clásica de las ondas, la energía de una onda depende de su intensidad (que depende de su amplitud), no de su frecuencia. Una parte de estas observaciones fue que el número de electrones expulsados dentro de un período de tiempo determinado aumentó a medida que el brillo aumentaba. En 1905, Albert Einstein fue capaz de resolver la paradoja incorporando los hallazgos de cuantificación de Planck en la desacreditada visión de las partículas de la luz.
 
 Einstein argumentó que las energías cuantificadas que Planck había postulado podrían ser aplicadas a la luz en el efecto fotoeléctrico. La luz que golpea la superficie metálica no debe ser vista como una onda, sino como una corriente de partículas (más tarde llamadas fotones) cuya energía dependía de su frecuencia. La cantidad de energía (E) en un paquete de luz depende de su frecuencia (ν) de acuerdo con la siguiente ecuación:
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/976/976_Equations-01.png" style="height: 40px;" />
 
 Donde h es la constante de Planck.
 
 El efecto fotoeléctrico se puede describir asumiendo que la luz es cuantificada. Se requiere una cierta energía mínima para superar la energía de unión (Φ) experimentada por un electrón. Esto también se conoce como la función de trabajo (W) del metal.
 
 Puesto que los electrones en el metal tenían una cierta cantidad de energía de unión manteniéndolos allí, la luz incidente necesita tener más energía para liberar a los electrones. Los fotones de luz de baja frecuencia no contienen suficiente energía para expulsar electrones del metal. Incluso si el metal está expuesto a tal luz durante mucho tiempo, no se observa ninguna emisión de electrones. Un electrón sólo puede ser emitido cuando un fotón con energía mayor que la función de trabajo golpea el metal.
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/976/976_Equations-02.png" style="height: 40px;" />
 
 El exceso de energía del fotón se convierte en energía cinética del electrón emitido.
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/976/976_Equations-03.png" style="height: 40px;" />
 
 Por lo tanto, los electrones son expulsados cuando son golpeados por fotones que tienen suficiente energía (una frecuencia mayor que el umbral). Cuanto mayor sea la frecuencia de la luz incidente, mayor será la energía cinética impartida por las colisiones a los electrones que escapan. Einstein también sostuvo que la intensidad de la luz no dependía de la amplitud de la onda entrante, sino que correspondía al número de fotones que golpeaban la superficie dentro de un período de tiempo dado. El número de electrones expulsados aumenta con el brillo. Cuanto mayor sea el número de fotones entrantes, más probable será que colisionen con algunos de los electrones.
 
 El efecto fotoeléctrico sugiere fuertemente el comportamiento de la luz como partícula. Einstein ganó el Premio Nobel de Física en 1921 por su explicación del efecto fotoeléctrico. Aunque muchos fenómenos de luz podrían explicarse en términos de ondas o partículas, ciertos fenómenos, como los patrones de interferencia obtenidos cuando la luz pasaba a través de una doble rendija, eran completamente contrarios a una visión de la luz como partículas, mientras que otros fenómenos, como el efecto fotoeléctrico, eran completamente contrarios a una visión de la luz como onda . De alguna manera, a un nivel fundamental profundo aún no completamente entendido, la luz es a la vez ondulante y parecida a partículas. Esto se conoce como dualidad onda-partícula.
 
 Este texto está adaptado de <a href="https://openstax.org/books/chemistry-2e/pages/6-1-electromagnetic-energy">Openstax, Química 2e, Sección 6.1: Energía electromagnética</a>.

El Efecto Fotoeléctrico

Cuando la luz de una longitud de onda particular incide sobre una superficie metálica, se emiten electrones. A esto se le llama efecto fotoeléctrico. La frecuencia 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

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

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Louisiana Student Standards for Science

High School

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Motion And Stability: Forces And Interactions

HS-PS2-6

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Ohio’s Learning Standards and Model Curriculum

Physics

Waves

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Tennessee Academic Standards

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Waves and Their Applications in Technologies for Information Transfer

PHYS.PS4.7

CHEM1.PS4.1

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HS+C.P1

HS+C.P1U1.2

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SC.912.P.8

SC.912.P.8.3

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PHYS.9

PHYS.9.A

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South Carolina College and Career Ready Science Standards

C-PS4-5

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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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