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

Belirli bir dalga boyu ve frekanstaki ışık bir metale çarptığında, elektronlar metalden koparılır. Bu fenomen, fotoelektrik etki olarak bilinir. Bununla birlikte, yalnızca bir eşik frekansının üzerindeki ışınlar elektronları metalden fırlatabilir.Daha düşük frekanslı ışık, yoğunluğu ne olursa olsun elektronları fırlatmaz. Bu neden böyledir? Albert Einstein, ışığın bir parçacık akışı veya küçük paketler gibi davrandığını öne sürmüştür.Bir birim ışık foton olarak adlandırılır. Her fotonun frekansı yani nu'ya bağlı olan bir enerjisi vardır. Bu ikisi, h'nin Planck sabiti olduğu ve 6, 626 çarpı 10 üzeri eksi 34 jul-saniye değerine sahip olduğu bu denklemle ilişkilidir.Daha yüksek frekanslı ışık ve dolayısıyla fotonlar, daha büyük enerjiye sahiptir. Bir elektron, bağlanma enerjisi phi'ile bir metale bağladır. Bu aynı zamanda metalin iş fonksiyonu W olarak da bilinir.Bu nedenle, çekici kuvvetlerin üstesinden gelmek ve elektronu yerinden koparmak için W'dan daha büyük bir enerji miktarı gereklidir. W'dan daha büyük enerjiye sahip bir foton metale çarptığında, fazla enerjisi kinetik enerji olarak elektrona aktarılır ve elektron dışarı atılır. Fotoelektrik etkiyi özetlemek gerekirse, düşük frekanslı, uzun dalga boylu ışık metalden herhangi bir elektron çıkaramaz.Düşük frekanslı ışığın yoğunluğunu artırmak yalnızca foton sayısını artırır. Her foton hala aynı enerjiye sahiptir ve bu da elektronları yerinden oynatmak için yeterli değildir. Eşik frekansından daha yüksek frekansa sahip daha kısa dalga boylu ışık, elektronları fırlatır çünkü bu durumda her bir foton daha yüksek enerjiye sahiptir.Işığın frekansı arttıkça, fotonun enerjisi ve dolayısıyla fırlatılan elektronun kinetik enerjisi de artar. Işığın yoğunluğu arttıkça, daha fazla foton metale çarpar ve atılan elektron sayısı da artar. Fotoelektrik etki, ışığın parçacık davranışını gösterir.

fotoelektrik olay

Belirli bir dalga boyundaki ışık metal bir yüzeye çarptığında, elektronlar yayılır. Buna fotoelektrik olay denir. Bu tür elektron emisyonlarına neden olabilecek minimum ışık frekansına metale özgü eşik frekansı denir. Eşik frekansından daha düşük frekanslı ışık, yüksek yoğunlukta olsa bile elektron emisyonunu başlatamaz. Bununla birlikte, frekans eşik değerden yüksek olduğunda, atılan elektron sayısı doğrudan ışının yoğunluğu ile orantılıdır.
 
 Klasik dalga teorisine göre, bir dalganın enerjisi frekansına değil yoğunluğuna (genliğine bağlıdır) bağlıdır. Bu gözlemlerin bir kısmı, belirli bir süre içinde atılan elektron sayısının parlaklık arttıkça arttığı görülmesiydi. 1905'te Albert Einstein, Planck'ın nicemleme bulgularını ışığın gözden düşmüş parçacık görünümüne dahil ederek paradoksu çözmeyi başardı.
 
 Einstein, Planck'ın varsaydığı nicelenmiş enerjilerin fotoelektrik etkideki ışığa uygulanabileceğini savundu. Metal yüzeye çarpan ışık bir dalga olarak görülmemeli, bunun yerine enerjisi frekanslarına bağlı olan bir parçacık akışı (daha sonra fotonlar olarak adlandırılır) olarak görülmelidir.Bir ışık paketindeki enerji miktarı (E) aşağıdaki denkleme göre frekansına (ν) bağlıdır:
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/976/976_Equations-01.png" style="height: 40px;" />
 
 where h Planck sabitidir.
 
 Fotoelektrik olay, ışığın nicemlendiğini varsayarak tanımlanabilir. Bir elektron tarafından deneyimlenen bağlanma enerjisinin (Φ) üstesinden gelmek için belirli bir minimum enerji gerekir. Bu aynı zamanda metalin iş fonksiyonu (W) olarak da bilinir.
 
 Metaldeki elektronlar, onları orada tutan belirli miktarda bağlanma enerjisine sahip olduklarından, çarpan ışığının elektronları serbest bırakmak için daha fazla enerjiye sahip olması gerekir. Düşük frekanslı ışığın fotonları, elektronları metalden fırlatmaya yetecek kadar enerji içermez. Metal uzun süre böyle bir ışığa maruz kalsa bile elektron emisyonu gözlenmez. Bir elektron ancak iş fonksiyonundan daha büyük enerjiye sahip bir foton metale çarptığında yayılabilir.
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/976/976_Equations-02.png" style="height: 40px;" />
 
 Fotonun fazla enerjisi, yayılan elektronun kinetik enerjisine dönüştürülür.
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/976/976_Equations-03.png" style="height: 40px;" />
 
 Bu nedenle, elektronlar, yeterli enerjiye (eşikten daha büyük bir frekansa) sahip fotonlar tarafından vurulduğunda dışarı atılır. Gelen ışığın frekansı ne kadar büyükse, kaçan elektronlara çarpışmaların verdiği kinetik enerji o kadar büyük olur. Einstein ayrıca ışık yoğunluğunun gelen dalganın genliğine bağlı olmadığını, bunun yerine belirli bir süre içinde yüzeye çarpan fotonların sayısına karşılık geldiğini savundu. Püskürtülen elektronların sayısı parlaklıkla artar. Gelen fotonların sayısı arttıkça, bazı elektronlarla çarpışmaları daha olasıdır.
 
 Fotoelektrik olay güçlü bir şekilde ışığın parçacık davranışını gösterir. Einstein, fotoelektrik olayı açıklaması nedeniyle 1921'de Nobel Fizik Ödülü'nü kazandı. Birçok ışık fenomeni dalgalar veya partiküller olarak açıklanabilse de, ışık çift yarıktan geçerken elde edilen girişim desenleri gibi bazı fenomenler, ışığın partikül görünümüne tamamen aykırı iken, fotoelektrik gibi diğer fenomenler etkisi, ışığın dalga görünümüne tamamen aykırıdır. Her nasılsa, derin bir temel seviyede hala tam olarak anlaşılamamıştır, ışık hem dalgalı hem de parçacık benzeridir. Bu dalga-parçacık ikiliği olarak bilinir.
 
 Bu metin bu kaynaktan uyarlanmıştır: <a href="https://openstax.org/books/chemistry-2e/pages/6-1-electromagnetic-energy">Openstax, Chemistry 2e, Section 6.1: Electromagnetic Energy</a>.

Fotoelektrik Olay

Belirli bir dalga boyundaki ışık metal bir yüzeye çarptığında elektronlar yayılır. Buna fotoelektrik etki denir. Minimum frekansı ...

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Electronic Structure of Atoms

Atomların Elektronik Yapısı

KEY TERMS AND CONCEPTS

TEMEL TERİMLER VE KAVRAMLAR

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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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

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emission spectra

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the quantum-mechanical model of an atom

Electron Orbitals in Quantum Mechanics

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the aufbau principle and hund's rule

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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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