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

Wanneer licht van een bepaalde golflengte en frequentie een metaal raakt, worden elektronen uit het metaal geworpen. Dit fenomeen staat bekend als het foto-elektrisch effect. Alleen licht boven een drempelfrequentie kan echter elektronen uit het metaal werpen.Licht met een lagere frequentie werpt geen elektronen uit ongeacht de intensiteit. Hoe komt dit? Albert Einstein stelde voor dat licht zich gedraagt als een stroom deeltjes of kleine pakketjes.Een pakket of quantum licht wordt een foton genoemd. Elk foton heeft een energie, E, die afhangt van zijn frequentie, nu. De twee zijn gerelateerd door deze vergelijking, waarbij h de constante van Planck is en een waarde heeft van 6, 626 maal 10 tot de negatieve 34 joule-seconden.Licht, en dus fotonen, met een hogere frequentie hebben meer energie. Een elektron is gebonden aan een metaal met een bindingsenergie, phi. Dit staat ook bekend als de werkfunctie van het metaal, W.Er is dus een hoeveelheid energie groter dan W nodig om de aantrekkingskrachten te overwinnen en het elektron vrij te maken.Wanneer een foton met een energie groter dan W het metaal raakt, wordt zijn overtollige energie als kinetische energie overgedragen op het elektron en wordt het elektron uitgeworpen. Om het foto-elektrische effect samen te vatten, laagfrequent licht met een lange golflengte werpt geen elektronen uit het metaal. Door de intensiteit van laagfrequent licht te verhogen, neemt het aantal fotonen alleen maar toe.Elk foton heeft nog steeds dezelfde energie, wat niet genoeg is om elektronen vrij te maken. Licht met een kortere golflengte met een frequentie die hoger is dan de drempelfrequentie, werpt elektronen uit omdat elk foton een hogere energie heeft. Naarmate de frequentie van licht toeneemt, neemt ook de energie van het foton toe, en dus de kinetische energie van het uitgeworpen elektron.Hoe groter de intensiteit van het licht, hoe meer fotonen het metaal raken en hoe groter het aantal elektronen dat wordt uitgeworpen. Het foto-elektrische effect toont het deeltjesgedrag van licht aan.

fotoëlektrisch effect

Wanneer licht van een bepaalde golflengte op een metalen oppervlak valt, worden elektronen uitgezonden. Dit wordt het foto-elektrisch effect genoemd. De minimale lichtfrequentie die een dergelijke emissie van elektronen kan veroorzaken, wordt de drempelfrequentie genoemd, die specifiek is voor het metaal. Licht met een frequentie lager dan de drempelfrequentie kan, zelfs als deze een hoge intensiteit heeft, de emissie van elektronen niet initiëren. Wanneer de frequentie echter hoger is dan de drempelwaarde, is het aantal uitgestoten elektronen recht evenredig met de intensiteit van de straal. Volgens de klassieke golftheorie hangt de energie van een golf af van de intensiteit (die afhangt van de amplitude), en niet van de frequentie. Eén deel van deze waarnemingen was dat het aantal elektronen dat binnen een bepaalde tijdsperiode werd uitgestoten, toenam naarmate de helderheid toenam. In 1905 slaagde Albert Einstein erin de paradox op te lossen door de kwantiseringsbevindingen van Planck op te nemen in de in diskrediet gebrachte deeltjesvisie van licht. Einstein voerde aan dat de gekwantiseerde energieën die Planck had gepostuleerd, konden worden toegepast op het licht in het foto-elektrische effect. Het licht dat op het metalen oppervlak valt, moet niet worden gezien als een golf, maar moet in plaats daarvan worden gezien als een stroom deeltjes (later fotonen genoemd) waarvan de energie afhangt van hun frequentie. De hoeveelheid energie (E) in een lichtpakketje hangt af van de frequentie ervan. frequentie (ν) volgens de volgende vergelijking: <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/976/976_Equations-01.png" style="height: 40px;"> waarbij h de constante van Planck is. Het foto-elektrische effect kan worden beschreven door aan te nemen dat het licht gekwantiseerd is. Er is een bepaalde minimale energie nodig om de bindingsenergie (Φ) te overwinnen die een elektron ervaart. Dit wordt ook wel de werkfunctie (W) van het metaal genoemd. Omdat de elektronen in het metaal een bepaalde hoeveelheid bindingsenergie hadden die ze daar vasthield, heeft het invallende licht meer energie nodig om de elektronen vrij te maken. Fotonen van laagfrequent licht bevatten niet genoeg energie om elektronen uit het metaal te werpen. Zelfs als het metaal langdurig aan dergelijk licht wordt blootgesteld, wordt er geen emissie van elektronen waargenomen. Een elektron kan alleen worden uitgezonden als een foton met een energie groter dan de werkfunctie het metaal raakt. <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/976/976_Equations-02.png" style="height: 40px;"> De overtollige energie van het foton wordt omgezet in kinetische energie van het uitgezonden elektron. <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/976/976_Equations-03.png" style="height: 40px;"> Daarom worden elektronen uitgestoten wanneer ze worden geraakt door fotonen met voldoende energie (een frequentie groter dan de drempelwaarde). Hoe groter de frequentie van invallend licht, hoe groter de kinetische energie die door de botsingen wordt verleend aan de ontsnappende elektronen. Einstein voerde ook aan dat de lichtintensiteit niet afhankelijk was van de amplitude van de binnenkomende golf, maar in plaats daarvan correspondeerde met het aantal fotonen dat binnen een bepaalde tijdsperiode het oppervlak bereikte. Het aantal uitgestoten elektronen neemt toe met de helderheid. Hoe groter het aantal binnenkomende fotonen, hoe waarschijnlijker het is dat ze met een deel van de elektronen zullen botsen. Het foto-elektrische effect suggereert sterk het deeltjesgedrag van licht. Einstein won in 1921 de Nobelprijs voor de natuurkunde voor zijn verklaring van het foto-elektrische effect. Hoewel veel lichtverschijnselen konden worden verklaard in termen van golven of deeltjes, waren bepaalde verschijnselen, zoals de interferentiepatronen die werden verkregen wanneer licht door een dubbele spleet ging, volledig in tegenspraak met de deeltjesvisie van licht, terwijl andere verschijnselen, zoals de foto-elektrische effect, waren volledig in strijd met een golfweergave van licht. Op een of andere manier, op een diep fundamenteel niveau dat nog steeds niet volledig wordt begrepen, is licht zowel golfachtig als deeltjesachtig. Dit staat bekend als golf-deeltjes dualiteit. Deze tekst is aangepast van <a href="https://openstax.org/books/chemistry-2e/pages/6-1-electromagnetic-energy">Openstax, Chemistry 2e, Sectie 6.1: Elektromagnetische Energie</a>.

Fotoëlektrisch effect

When light of a particular wavelength strikes a metal surface, electrons are emitted. This is called the photoelectric effect. The minimum frequency of ...

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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,...

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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...

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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Why Light Ejects Electrons from Metals

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