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

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Physical Science

Waves and Electromagnetic Radiation

3.2.9-12.X

Photoelectric Effect

Wenn Licht einer bestimmten Wellenlänge und Frequenz auf ein Metall trifft, werden Elektronen aus dem Metall herausgeschleudert. Dieses Phänomen ist als photoelektrischer Effekt bekannt. Allerdings kann nur Licht oberhalb einer bestimmten Frequenz Elektronen aus dem Metall herausschleudern.Licht mit niedrigen Frequenzen stößt keine Elektronen aus, unabhängig von seiner Intensität. Wie ist das möglich? Albert Einstein schlug die Theorie vor, dass das Licht sich wie ein Strom von Teilchen oder winzigen Paketen verhält.Ein Paket oder Quantum von Licht wird Photon genannt. Jedes Photon hat eine Energie E die von seiner Frequenz nu abhängt. Die beiden sind durch diese Gleichung miteinander verbunden, wobei h die Planck'sche Konstante ist und hat einen Wert von 6, 626 mal 10 hoch 34 Joule Sekunden.Licht, und damit Photonen, mit höherer Frequenz haben mehr Energie. Ein Elektron ist an ein Metall mit einer Bindungsenergie gebunden. Dies ist auch bekannt als die Arbeitsfunktion des Metalls, W.Also, eine Energiemenge größer als W ist erforderlich, um die Anziehungskräfte zu überwinden und das Elektron zu verdrängen.Wenn ein Photon mit einer Energie größer als W auf das Metall trifft, wird seine überschüssige Energie als kinetische Energie auf das Elektron übertragen und das Elektron wird ausgestoßen. Um den photoelektrischen Effekt zusammenfassen langwelliges Licht mit niedriger Frequenz stößt keine Elektronen aus dem Metall aus. Erhöhung der Intensität von langwelligem Licht erhöht nur die Anzahl der Photonen.Jedes Photon hat immer noch die gleiche Energie, was nicht ausreicht, um irgendwelche Elektronen zu verdrängen. Licht kürzerer Wellenlänge mit einer Frequenz höher als die Grenzfrequenz wirft Elektronen aus, weil jedes Photon eine höhere Energie hat. Wenn die Frequenz des Lichts zunimmt, so auch die Energie des Photons, und damit die kinetische Energie des ausgestoßenen Elektrons.Je größer die Lichtintensität, desto mehr Photonen treffen auf das Metall, und desto größer ist die Anzahl der Elektronen, die ausgestoßen werden. Der photoelektrische Effekt zeigt das Teilchenverhalten von Licht.

photoelektrischer effekt

Wenn Licht einer bestimmten Wellenlänge auf eine Metalloberfläche trifft, werden Elektronen emittiert. Dies wird als photoelektrischer Effekt bezeichnet. Die minimale Frequenz des Lichts, das eine solche Elektronenemission verursachen kann, wird als Schwellenfrequenz bezeichnet und ist für das Metall spezifisch. Licht mit einer niedrigeren Frequenz kann, selbst wenn es von hoher Intensität ist, nicht die Emission von Elektronen auslösen. Wenn die Frequenz jedoch höher als der Schwellenwert ist, wird die Anzahl der ausgestoßenen Elektronen direkt proportional zur Intensität des Strahls. Nach der klassischen Wellentheorie hängt die Energie einer Welle von ihrer Intensität (die von ihrer Amplitude abhängt) und nicht von ihrer Frequenz ab. Ein Teil dieser Beobachtungen bestand darin, dass die Anzahl der innerhalb eines bestimmten Zeitraums ausgestoßenen Elektronen mit zunehmender Helligkeit zunahm. Im Jahr 1905 gelang es Albert Einstein, das Paradox zu lösen, indem er die Forschungsergebnisse Planck's in die damals diskreditierte Theroie des Lichts als Teilchen einbezog. Einstein argumentierte, dass die von Planck postulierten quantisierten Energien auf das Licht im photoelektrischen Effekt angewendet werden könnten. Das auf die Metalloberfläche auftreffende Licht sollte nicht als Welle, sondern als Strom von Teilchen (später Photonen genannt) betrachtet werden, deren Energie von ihrer Frequenz abhängt. Die Energiemenge (E) in einem Lichtpaket hängt somit von ihrer Frequenz (ν) ab gemäß der folgenden Gleichung: <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/976/976_Equations-01.png" style="height: 40px;"> wobei h das Plancksche Wirkungsquantum ist. Der photoelektrische Effekt kann durch die Annahme beschrieben werden, dass das Licht quantisiert ist. Um die Bindungsenergie (Φ) eines Elektrons zu überwinden, ist eine bestimmte Mindestenergie erforderlich. Dies wird auch als die Austrittsarbeit (W) des Metalls bezeichnet. Da die Elektronen im Metall durch eine bestimmte Bindungsenergie gebunden sind, muss das einfallende Licht eine höhere Energie besitzen um die Elektronen freizusetzen. Photonen niederfrequenten Lichts enthalten nicht genug Energie, um Elektronen aus dem Metall auszustoßen. Selbst wenn das Metall über einen längeren Zeitraum diesem Licht ausgesetzt wird, wird keine Emission von Elektronen beobachtet. Ein Elektron kann nur emittiert werden, wenn ein Photon mit einer Energie größer als die Austrittsarbeit auf das Metall trifft. <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/976/976_Equations-02.png" style="height: 40px;"> Die überschüssige Energie des Photons wird in kinetische Energie des emittierten Elektrons umgewandelt. <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/976/976_Equations-03.png" style="height: 40px;"> Daher werden Elektronen ausgestoßen, wenn sie von Photonen mit ausreichender Energie (einer Frequenz über dem Schwellenwert) getroffen werden. Je höher die Frequenz des einfallenden Lichts, desto größer ist die kinetische Energie, die durch die Kollisionen auf die austretenden Elektronen übertragen wird. Einstein argumentierte auch, dass die Lichtintensität nicht von der Amplitude der einfallenden Welle abhänge, sondern vielmehr der Anzahl der Photonen entspreche, die innerhalb eines bestimmten Zeitraums auf die Oberfläche auftreffen. Die Anzahl der ausgestoßenen Elektronen nimmt mit der Intensität zu. Je mehr Photonen einfallen, desto wahrscheinlicher ist es, dass sie auch mit einigen Elektronen kollidieren. Der photoelektrische Effekt ist ein wichtiges Beispiel für das Teilchenverhalten von Licht. Für seine Erklärung des photoelektrischen Effekts erhielt Einstein 1921 den Nobelpreis für Physik. Obwohl viele Lichtphänomene entweder durch seine Wellen- oder Teilchennatur erklärt werden konnten, widersprachen bestimmte Phänomene, wie etwa die Interferenzmuster die beim Durchgang von Licht durch einen Doppelspalt entstehen, völlig der Sicht von Licht als Teilchen während andere Phänomene, wie etwa die Photoelektrizität, entgegen der Betrachtung von Licht als Welle stehen. Somit folgt Licht, auf einer fundamentalen Ebene die immer noch nicht vollständig verstanden wird, sowohl dem Verhalten von Wellen als auch Teilchen. Dies wird als Welle-Teilchen-Dualität bezeichnet. Dieser Text wurde angepasst von <a href="https://openstax.org/books/chemistry-2e/pages/6-1-electromagnetic-energy">Openstax, Chemistry 2e, Section 6.1: Electromagnetic Energy</a>.

Photoelektrischer Effekt

Wenn Licht einer bestimmten Wellenlänge auf eine Metalloberfläche trifft, werden Elektronen emittiert. Dies wird als photoelektrischer Effekt bezeichnet. Die Mindesthäufigkeit von ...

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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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interference and diffraction

Wave Patterns from Light and Slits

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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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the quantum-mechanical model of an atom

Electron Orbitals in Quantum Mechanics

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atomic orbitals

Electron Regions in Atomic Orbitals

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the pauli exclusion principle

Electron Spin and Orbital Pairing

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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

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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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