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

特定の波長や周波数の 光が金属に当たると 金属から電子が 放出されます この現象を光電効果といいます しかし 金属から電子が 飛び出すことができるのは ある閾値周波数以上の 光の場合だけです 周波数の低い光は その強度にかかわらず 電子を追い出すことが できません これはどうして そうなるのでしょうか？アルバート・アインシュタインは 光が粒子や小さなパケットの 流れとして 振る舞うことを提案しました 1つのパケットまたは量子は 光子と呼ばれています 各光子はエネルギー Eを持ち それは周波数 νに依存します この2つはこの式によって 関係しており hはプランク定数であり 6.626x10のマイナス34乗 ジュール秒 の値を持っています 光 つまり光子は より高い周波数のものほど 大きなエネルギーを持っています 電子は金属と結合エネルギー ファイで結合しています これは金属の 仕事関数 Wとして知られています そのため 電子の 吸引力に打ち勝って 電子を取り除くためには Wよりも大きな エネルギーが必要となります W以上のエネルギーを 持つ光子が金属に当たると その余剰エネルギーが 運動エネルギーとして 電子に伝わり 電子が放出されます 光電効果を要約すると 低周波の長波長光では 金属から電子は放出されません 低周波光の強度を上げると 光子の数が増えるだけです 1つ1つの光子の エネルギーは同じですが 電子を放出するには 十分ではありません しきい値周波数よりも高い 周波数の短い波長の光は 光子のエネルギーが高いため 電子を放出させます 光の周波数が高くなると 光子のエネルギーも高くなり 放出される電子の運動 エネルギーも高くなります 光の強度が高ければ高いほど より多くの光子が金属に当たり 放出される 電子の数が多くなります 光電効果は 光の 粒子挙動を示しています

光電効果

金属の表面に特定の波長の光が当たると、電子が放出されます。これを光電効果といいます。 このような電子の放出を起こすことができる光の最小周波数をしきい値周波数といい、金属に固有のものです。 しきい周波数よりも低い周波数の光は、たとえ強度が高くても電子の放出を開始することはできません。 しかし、周波数がしきい値よりも高い場合には、放出される電子の数はビームの強度に正比例します。 
 
 古典的な波動理論では、波のエネルギーは周波数ではなく強度（振幅）に依存します。 これらの観測結果の中には、一定時間内に放出される電子の数が、明るさが増すにつれて増加するというものがありました。 1905年、アルバート・アインシュタインは、プランクの量子化の発見を、否定されていた粒子論的な光の見方に組み込むことで、パラドックスを解決することができました。 
 
 アインシュタインは、プランクが仮定した量子化エネルギーを光電効果の光にも応用できると主張しました。 金属表面に当たった光を波と見なすのではなく、エネルギーが周波数に依存する粒子（後に光子と呼ばれる）の流れと見なすべきです。 ある光のエネルギー量（ E ）は、次式のように周波数（ν）に依存します。 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/976/976_Equations-01.png" style="height: 40px;" />
 
 h はプランク定数です。 
 
 光電効果は、光が量子化されていると仮定することで説明できます。 電子がもつ結合エネルギー（ Φ ）を克服するには、一定の最小エネルギーが必要です。 これは、金属の仕事関数（ W）と呼ばれます。 
 
 金属中の電子にはある程度の結合エネルギーがあるので、電子を解放するためには、入射光にもっと大きなエネルギーが必要になります。 低周波の光には、金属から電子を放出させるだけのエネルギーがありません。このような光を長時間照射しても、電子の放出は見られません。 仕事関数以上のエネルギーを持つ光子が金属に衝突して初めて電子が放出されます。 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/976/976_Equations-02.png" style="height: 40px;" />
 
 光子の過剰エネルギーは、放出された電子の運動エネルギーに変換されます。 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/976/976_Equations-03.png" style="height: 40px;" />
 
 そのため、十分なエネルギー（しきい値を超える周波数）を持つ光子が当たると、電子が放出されます。 入射光の周波数が高いほど、放出される電子との衝突によって生じます。運動エネルギーが大きくなります。 アインシュタインはまた、光強度は入射波の振幅に依存していないが、一定時間内に表面に衝突した光子の数に依存すると主張しました。 放出される電子の数は、輝度とともに増加します。 入射光子の数が多いほど、一部の電子と衝突する可能性が高くなります。 
 
 光電効果は、光の粒子的な振る舞いを強く示唆するものです。 アインシュタインは、 1921 年に光電効果の説明でノーベル物理学賞を受賞しました。 多くの光の現象は波でも粒子でも説明できるが、二重スリットを通過したときに得られる干渉縞のように、粒子的な光の見方に完全に反する現象もあれば、光電効果のように波的な光の見方に完全に反する現象もあります。 まだ完全には解明されていない深い基本的なレベルで、光は波動的であると同時に粒子的でもあるといえます。 これを、粒子と波動の二重性と呼びます。 
 
 本書は <a href="https://openstax.org/books/chemistry-2e/pages/6-1-electromagnetic-energy">、 Openstax 、 Chemistry 2e 、 Section 6.1 ： Electromagnetic Energy から引用</a>しています。

特定の波長の光が金属表面に当たると、電子が放出されます。これを光電効果といいます。最小周波数 ...

Education

JoVE Core

Chemistry

化学

Electronic Structure of Atoms

原子の電子構造

KEY TERMS AND CONCEPTS

主要用語と概念

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

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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

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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

chemistry

JoVE Core 化学は、簡潔で理解しやすいアニメーションビデオレッスンを使用して、一般的な化学の概念を生き生きとしたものにします。さらに、活動する科学者のビデオでは、現在世界中の実験室で行われている古典的そして独創的な研究・実験をお見せします。

Louisiana Student Standards for Science

High School

motion-and-stability-forces-and-interactions

Motion And Stability: Forces And Interactions

HS-PS2-6

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Ohio’s Learning Standards and Model Curriculum

Physics

Waves

Light phenomena

Tennessee Academic Standards

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Waves and Their Applications in Technologies for Information Transfer

PHYS.PS4.7

CHEM1.PS4.1

Arizona Science Standards

HS+C.P1

HS+C.P1U1.2

Next Generation Science Standards

HS-PS4-4

Florida Department of Education

SC.912.P.8

SC.912.P.8.3

Texas Essential Knowledge and Skills

PHYS.9

PHYS.9.A

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South Carolina College and Career Ready Science Standards

C-PS4-5

new-york-state-science-learning-standards

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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