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

특정한 파장과 진동수의 빛이 금속과 부딪칠 때 전자가 방출됩니다. 이 현상은 광전 효과로 알려져 있습니다. 그러나 한계 진동수 이상의 빛만이 금속에서 전자를 방출할 수 있습니다.

진동수가 낮은 빛은 그 세기와 관계없이 전자를 방출하지 않습니다. 왜 그럴까요? 알버트 아인슈타인은 빛은 입자의 흐름이나 작은 꾸러미처럼 작용한다고 제안했습니다.

빛의 한 묶음을 광자라고 부릅니다. 광자는 진동수, 뉴에 따라 달라지는 에너지 E를 가지고 있습니다. 두 변수는 이 방정식에 의해 관련되는데, 여기서 h는 플랑크 상수로서 값이 6.626 곱하기 10의 마이너스 34승 줄 퍼 세컨드입니다.

높은 진동수의 빛은, 즉 광자는 더 큰 에너지를 가지고 있습니다. 전자는 결합 에너지 파이 를 가지고 금속에 결합되어 있습니다. 이것은 또한 금속의 일함수 W라고 알려져 있습니다.

따라서 인력을 극복하고 전자를 벗어나게 하려면 W보다 더 큰 에너지가 필요합니다. W보다 큰 에너지를 가진 광자가 금속과 부딪칠 때, 잉여 에너지는 운동 에너지로 전자에 전달되고 전자는 방출됩니다. 광전 효과를 요약하면 낮은 진동수, 긴 파장의 빛은 금속에서 어떤 전자도 방출하지 못합니다.

낮은 진동수의 빛의 세기를 높이면 광자의 수만 증가할 뿐입니다. 각 광자는 여전히 같은 에너지를 가지고 있고, 이것은 어떤 전자도 떼어낼 수 없습니다. 한계 진동수보다 높은 진동수를 가진 짧은 파장의 빛은 매 광자가 높은 에너지를 가지기 때문에 전자를 방출합니다.

빛의 진동수가 증가함에 따라 광자의 에너지도 증가하게 되고, 따라서 방출된 전자의 운동 에너지도 증가하게 됩니다. 빛의 세기가 클수록 더 많은 광자가 금속을 타격하고, 방출되는 전자의 수도 더 많아집니다. 광전 효과는 빛의 입자의 성격을 보여줍니다.

광전 효과

특정 파장의 빛이 금속 표면에 닿으면 전자가 방출됩니다. 이것을 광전 효과라고 합니다. 이러한 전자 방출을 일으킬 수 있는 빛의 최소 주파수를 임계 주파수라고 하며 이는 금속에 따라 다릅니다. 임계 주파수보다 낮은 주파수를 갖는 빛은 강도가 높더라도 전자 방출을 시작할 수 없습니다. 그러나 주파수가 임계값보다 높으면 방출되는 전자의 수는 빔의 강도에 정비례합니다.

고전파 이론에 따르면 파동의 에너지는 주파수가 아니라 강도(진폭에 따라 다름)에 따라 달라집니다. 이러한 관찰의 한 부분은 주어진 기간 내에 방출되는 전자의 수가 밝기가 증가함에 따라 증가하는 것으로 나타났다는 것입니다. 1905년에 알베르트 아인슈타인(Albert Einstein)은 플랑크의 양자화 발견을 신뢰할 수 없는 빛의 입자 관점에 통합하여 역설을 해결할 수 있었습니다.

아인슈타인은 플랑크가 가정한 양자화된 에너지가 광전 효과로 빛에 적용될 수 있다고 주장했습니다. 금속 표면에 닿는 빛은 파동으로 간주되어서는 안 되며, 그 대신 에너지가 주파수에 따라 달라지는 입자(이후 광자라고 함)의 흐름으로 보아야 합니다. 빛 패킷의 에너지량(E)은 다음 식과 같이 주파수(ν)에 따라 결정됩니다:

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/976/976_Equations-01.png" style="height: 40px;" />

여기서 h는 플랑크 상수입니다.

광전 효과는 빛이 양자화된다고 가정하여 설명할 수 있습니다. 전자가 경험하는 결합 에너지(Φ)를 극복하려면 특정 최소 에너지가 필요합니다. 이는 금속의 일함수(W)라고도 합니다.

금속의 전자는 특정 양의 결합 에너지를 가지고 있기 때문에 입사광은 전자를 자유롭게 하기 위해 더 많은 에너지를 가져야 합니다. 저주파 빛의 광자는 금속에서 전자를 방출할 만큼 충분한 에너지를 포함하지 않습니다. 금속이 이러한 빛에 장시간 노출되어도 전자 방출이 관찰되지 않습니다. 전자는 일함수보다 큰 에너지를 가진 광자가 금속에 부딪힐 때만 방출될 수 있습니다.

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/976/976_Equations-02.png" style="height: 40px;" />

광자의 과잉 에너지는 방출된 전자의 운동 에너지로 변환됩니다.

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/976/976_Equations-03.png" style="height: 40px;" />

따라서 전자는 충분한 에너지(임계값보다 큰 주파수)를 갖는 광자와 충돌할 때 방출됩니다. 입사광의 주파수가 클수록 충돌로 인해 탈출하는 전자에 전달되는 운동 에너지가 커집니다. 아인슈타인은 또한 빛의 강도는 들어오는 파동의 진폭에 의존하지 않고 대신 주어진 시간 내에 표면에 부딪히는 광자의 수에 상응한다고 주장했습니다. 방출된 전자의 수는 밝기에 따라 증가합니다. 들어오는 광자의 수가 많을수록 일부 전자와 충돌할 가능성이 높아집니다.

광전 효과는 빛의 입자 거동을 강력하게 암시합니다. 아인슈타인은 광전 효과에 대한 설명으로 1921년 노벨 물리학상을 수상했습니다. 많은 빛 현상은 파동이나 입자로 설명될 수 있지만, 빛이 이중 슬릿을 통과할 때 나타나는 간섭 무늬와 같은 특정 현상은 빛의 입자 관점과 완전히 반대되는 반면, 광전 현상과 같은 다른 현상은 효과는 빛의 파동 견해와 완전히 반대되었습니다. 어쨌든 아직 완전히 이해되지 않은 깊은 기본 수준에서 빛은 파동 같으면서도 입자 같습니다. 이것을 파동-입자 이중성이라고 합니다.

이 문서는 에서 발췌되었습니다 <a href="https://openstax.org/books/chemistry-2e/pages/6-1-electromagnetic-energy">Openstax, Chemistry 2e, Section 6.1: Electromagnetic Energy</a>.

특정 파장의 빛이 금속 표면에 부딪히면 전자가 방출됩니다. 이것을 광전 효과라고 합니다. 의 최소 빈도 ...

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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,...

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

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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3.2.9-12.V

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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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Dual Nature of Electromagnetic (EM) Radiation

Electromagnetic (EM) radiation consists of electric and magnetic field components oscillating in planes perpendicular to each other and mutually ...

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Interaction of EM Radiation with Matter: Spectroscopy

Electromagnetic (EM) radiation can be considered an oscillating electric and magnetic field propagating through a medium that can interact with matter in ...

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Molecular Spectroscopy: Absorption and Emission

Molecules possess discrete energy levels called quantum states. Unlike atoms, which have simpler energy levels, molecules possess additional rotational ...

Spectrophotometry: Introduction

Spectrophotometry is the quantitative measurement of the absorption, reflection, diffraction, or transmission of electromagnetic radiation through a ...

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Ultraviolet and Visible (UV&ndash;Vis) Spectroscopy: Overview

Ultraviolet&#8211;visible (UV&#8211;visible or UV&#8211;Vis) spectroscopy is an analytical technique that investigates the interaction between matter and ...

Infrared (IR) Spectroscopy: Overview

When electromagnetic radiation passes through a material, atoms or molecules transition from a lower to a higher energy state by absorbing radiation ...

Raman Spectroscopy: Overview

The underlying principle of Raman spectroscopy is based on the interaction between light and matter, specifically molecules&#39; inelastic scattering of ...

Atomic Absorption Spectroscopy: Overview

Atomic absorption spectroscopy (AAS) is a technique used to analyze elements by measuring electromagnetic radiation (EMR) absorbed by atoms, which causes ...

Nuclear Magnetic Resonance (NMR): Overview

Nuclear magnetic resonance (NMR) is a phenomenon exhibited by certain nuclei that can absorb characteristic radio frequency radiation under certain ...

Students who demonstrate understanding can communicate technical information about how some technological devices use the principles of wave behavior and wave interactions with matter to transmit and capture information and energy.  Clarifying Statement: Examples could include solar cells capturing light and converting it to electricity; medical imaging; and communications technology.  Assessment Boundry: Assessments are limited to qualitative information.  Assessments do not include band theory.

Why Light Ejects Electrons from Metals

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