Name: why light ejects electrons from metals
Uploaded: 2020-09-03T13:38:27.000Z
Duration: 2 min 26 s
Channel: JoVE

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Waves and Electromagnetic Radiation

3.2.9-12.X

Photoelectric Effect

Kiedy światło o określonej długości fali i częstotliwości uderza w metal, elektrony są wyrzucane z metalu. Zjawisko to znane jest jako efekt fotoelektryczny. 
Jednak tylko światło powyżej częstotliwości progowej może wyrzucać elektrony z metalu. Światło o niższej częstotliwości nie wyrzuca elektronów, niezależnie od jego natężenia. Jak to możliwe?
Albert Einstein zasugerował, że światło zachowuje się jak strumień cząstek lub maleńkich pakietów. Jeden pakiet – lub kwant – światła nazywa się fotonem.
Każdy foton ma energię E, która zależy od jego częstotliwości, ν. Oba są powiązane tym równaniem, gdzie h jest stałą Plancka i ma wartość 6,626 × 10−34 J⋅s. Światło, a co za tym idzie fotony o wyższej częstotliwości, mają większą energię.
Elektron jest związany z metalem o energii wiązania, phi. Jest to również znane jako funkcja robocza metalu, W. Kiedy foton o energii większej niż phi uderza w metal, jego nadmiar energii jest przekazywany elektronowi jako energia kinetyczna i elektron jest wyrzucany. Zatem energia kinetyczna elektronu jest różnicą między energią fotonu a energią wiązania elektronu.
Podsumowując efekt fotoelektryczny, światło o niskiej częstotliwości i długiej długości fali nie wyrzuca żadnych elektronów z metalu. Zwiększenie intensywności światła o niskiej częstotliwości zwiększa tylko liczbę fotonów. Każdy foton ma nadal tę samą energię, która nie wystarcza do usunięcia jakichkolwiek elektronów. 
Światło o krótszej długości fali o częstotliwości wyższej niż częstotliwość progowa wyrzuca elektrony, ponieważ każdy foton ma wyższą energię. Wraz ze wzrostem częstotliwości światła wzrasta energia fotonu, a co za tym idzie energia kinetyczna wyrzuconego elektronu. 
Im większe natężenie światła, tym więcej fotonów uderza w metal i tym większa liczba elektronów jest wyrzucana.
Efekt fotoelektryczny pokazuje zachowanie cząstek światła.

efekt fotoelektryczny

Kiedy światło o określonej długości fali uderza w metalową powierzchnię, emitowane są elektrony. Nazywa się to efektem fotoelektrycznym. Minimalna częstotliwość światła, która może spowodować taką emisję elektronów, nazywana jest częstotliwością progową, która jest specyficzna dla metalu. Światło o częstotliwości niższej od częstotliwości progowej, nawet jeśli jest duże, nie jest w stanie zainicjować emisji elektronów. Jeżeli jednak częstotliwość jest wyższa od wartości progowej, liczba wyrzuconych elektronów jest wprost proporcjonalna do intensywności wiązki.

Zgodnie z klasyczną teorią fal energia fali zależy od jej intensywności (która zależy od jej amplitudy), a nie od częstotliwości. Jedną z części tych obserwacji było to, że liczba elektronów wyrzuconych w danym okresie wzrastała wraz ze wzrostem jasności. W 1905 roku Albertowi Einsteinowi udało się rozwiązać ten paradoks, włączając ustalenia kwantyzacji Plancka do zdyskredytowanego poglądu na cząstki światła.

Einstein argumentował, że skwantowane energie, które postulował Planck, można zastosować do światła w efekcie fotoelektrycznym. Światło padające na powierzchnię metalu nie powinno być postrzegane jako fala, lecz jako strumień cząstek (później zwanych fotonami), których energia zależała od ich częstotliwości. Ilość energii (E) w pakiecie świetlnym zależy od jego częstotliwość (ν) zgodnie z następującym równaniem:

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/976/976_Equations-01.png" style="height: 40px;">

gdzie h jest stałą Plancka.

Efekt fotoelektryczny można opisać zakładając, że światło jest skwantowane. Aby pokonać energię wiązania (Φ) doświadczaną przez elektron, wymagana jest pewna minimalna energia. Jest to również znane jako funkcja pracy (W) metalu.

Ponieważ elektrony w metalu utrzymywały je w określonej ilości energii wiązania, padające światło musi mieć więcej energii, aby uwolnić elektrony. Fotony światła o niskiej częstotliwości nie zawierają wystarczającej energii, aby wyrzucić elektrony z metalu. Nawet jeśli metal jest wystawiony na działanie takiego światła przez długi czas, nie obserwuje się emisji elektronów. Elektron może zostać wyemitowany tylko wtedy, gdy foton o energii większej niż funkcja pracy uderza w metal.

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/976/976_Equations-02.png" style="height: 40px;">

Nadmiar energii fotonu przekształca się w energię kinetyczną wyemitowanego elektronu.

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/976/976_Equations-03.png" style="height: 40px;">

Elektrony są wyrzucane, gdy zostaną uderzone przez fotony o odpowiednio dużej energii (częstotliwość większa niż próg). Im większa częstotliwość padającego światła, tym większa energia kinetyczna przekazywana przez zderzenia uciekającym elektronom. Einstein argumentował również, że natężenie światła nie zależy od amplitudy nadchodzącej fali, ale odpowiada liczbie fotonów uderzających w powierzchnię w danym okresie. Liczba wyrzucanych elektronów wzrasta wraz ze wzrostem jasności. Im większa liczba przychodzących fotonów, tym większe prawdopodobieństwo, że zderzą się one z częścią elektronów.

Efekt fotoelektryczny silnie sugeruje zachowanie cząstek światła. Einstein otrzymał Nagrodę Nobla w dziedzinie fizyki w 1921 roku za wyjaśnienie efektu fotoelektrycznego. Chociaż wiele zjawisk świetlnych można wyjaśnić w kategoriach fal lub cząstek, pewne zjawiska, takie jak wzory interferencyjne powstające, gdy światło przechodzi przez podwójną szczelinę, są całkowicie sprzeczne z cząsteczkowym spojrzeniem na światło, podczas gdy inne zjawiska, takie jak zjawisko fotoelektryczne efektu, były całkowicie sprzeczne z falowym spojrzeniem na światło. W jakiś sposób, na głębokim, podstawowym poziomie, który wciąż nie jest w pełni poznany, światło jest zarówno falowe, jak i cząsteczkowe. Nazywa się to dualizmem korpuskularno-falowym.

Efekt fotoelektryczny

Kiedy światło o określonej długości fali uderza w metalową powierzchnię, emitowane są elektrony. Nazywa się to efektem fotoelektrycznym. Minimalna częstotliwość ...

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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

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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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