Name: why light ejects electrons from metals
Uploaded: 2020-09-03T13:38:27.000Z
Duration: 2 min 26 s
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K12 Standards

Pennsylvania State Academic Standards

9-12

Physical Science

Waves and Electromagnetic Radiation

3.2.9-12.X

Photoelectric Effect

כשאור באורך גל ובתדירות מסוימים מכה במתכת, אלקטרונים נפלטים מהמתכת. תופעה זו ידועה כאפקט הפוטואלקטרי. יחד עם זאת, רק אור מעל סף מסוים של תדירות יכול לגרום לפליטת אלקטרונים מהמתכת.אור בתדירות נמוכה יותר אינו גורם לפליטת אלקטרונים ללא קשר לעוצמתו. מדוע זה כך? אלברט איינשטיין טען שאור מתנהג כזרם של חלקיקים או כחבילות זעירות.חבילה אחת של אור, או קוונטום, נקראת פוטון. לכל פוטון יש אנרגיה, E, התלויה בתדירות שלו, nu. השניים קשורים זה לזה במשוואה זו, שבה h הוא קבוע של פלאנק ויש לו ערך של 6.626 כפול 10 במינוס 34 ג'אול-שניות.אור, ולכן גם פוטונים, בתדירות גבוהה יותר הם בעלי אנרגיה גדולה יותר. אלקטרון קשור למתכת עם אנרגית קישור, phi. הידועה גם בשם פונקציית העבודה של המתכת, W.לכן, כמות אנרגיה גבוה מפיי נדרשת על מנת להתגבר על כוחות המשיכה ולעקור את האלקטרון.כשפוטון עם אנרגיה גדולה מפיי מכה במתכת, האנרגיה העודפת שלו מועברת לאלקטרון כאנרגיה קינטית והאלקטרון נפלט. לסיכום האפקט הפוטואלקטרי, אור בתדר נמוך ובאורך גל ארוך אינו גורם לפליטת אלקטרונים מהמתכת. הגדלת העוצמה של אור בתדר נמוך רק מגדילה את מספר הפוטונים.לכל פוטון עדיין אותה אנרגיה, שאינה מספיקה לעקירת אלקטרונים. אור באורך גל קצר יותר בתדר גבוה מתדר הסף גורם לפליטת אלקטרונים, מפני שכל פוטון הוא בעל אנרגיה גבוהה יותר. ככל שתדירות האור עולה, כך גם האנרגיה של הפוטון, ולכן גם האנרגיה הקינטית של האלקטרון שנפלט.ככל שעוצמת האור גדולה יותר, פוטונים רבים יותר פוגעים במתכת ומספר גדול יותר של אלקטרונים נפלטים. האפקט הפוטואלקטרי מדגים את התנהגות החלקיקים של האור.

אפקט פוטואלקטרי

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 כאשר אור באורך גל מסוים פוגע במשטח מתכת, נפלטים אלקטרונים. זה נקרא האפקט הפוטואלקטרי. התדירות המינימלית של אור שיכולה לגרום לפליטת אלקטרונים כזו נקראת תדר הסף, שהוא ספציפי למתכת. אור עם תדר נמוך מתדר הסף, גם אם הוא בעוצמה גבוהה, אינו יכול ליזום פליטת אלקטרונים. עם זאת, כאשר התדר גבוה מערך הסף, מספר האלקטרונים הנפלטים עומד ביחס ישר לעוצמת הקרן. 

 לפי תורת הגלים הקלאסית, אנרגיית הגל תלויה בעוצמתו (התלויה במשרעתו), ולא בתדר שלו. חלק אחד מהתצפיות הללו היה שמספר האלקטרונים שנפלטו בפרק זמן נתון נראה גדל ככל שהבהירות גדלה. בשנת 1905, אלברט איינשטיין הצליח לפתור את הפרדוקס על ידי שילוב ממצאי הקוונטיזציה של פלאנק בתפיסת החלקיקים המוכפשת של האור. 

 איינשטיין טען שניתן להחיל את האנרגיות הקוונטיות שפלאנק הניח על האור באפקט הפוטואלקטרי. אין לראות את האור הפוגע במשטח המתכת כגל, אלא יש לראות בו כזרם של חלקיקים (שנקראו מאוחר יותר פוטונים) שהאנרגיה שלהם תלויה בתדירותם, כמות האנרגיה (E) בחבילת האור תלויה בה. תדר (ν) לפי המשוואה הבאה: <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/976/976_Equations-01.png" style="height: 40px;"> 

 כאשר h הוא הקבוע של פלאנק. 

 ניתן לתאר את האפקט הפוטואלקטרי על ידי הנחה שהאור הוא כמותי. נדרשת אנרגיה מינימלית מסוימת כדי להתגבר על אנרגיית הקישור (Φ) שחווה אלקטרון. זה ידוע גם כפונקציית העבודה (W) של המתכת. 

 מכיוון שלאלקטרונים במתכת הייתה כמות מסוימת של אנרגיית קשירה שהשאירה אותם שם, לאור הנכנס צריך להיות יותר אנרגיה כדי לשחרר את האלקטרונים. פוטונים של אור בתדר נמוך אינם מכילים מספיק אנרגיה כדי להוציא אלקטרונים מהמתכת. גם אם המתכת נחשפת לאור כזה במשך זמן רב, לא נצפית פליטת אלקטרונים. אלקטרון יכול להיפלט רק כאשר פוטון עם אנרגיה גדולה מתפקיד העבודה פוגע במתכת. <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/976/976_Equations-02.png" style="height: 40px;"> 

 האנרגיה העודפת של הפוטון מומרת לאנרגיה קינטית של האלקטרון הנפלט. <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/976/976_Equations-03.png" style="height: 40px;"> 

 לכן, אלקטרונים נפלטים כאשר הם נפגעים על ידי פוטונים בעלי אנרגיה מספקת (תדירות גבוהה מהסף). ככל שתדירות האור הנכנסת גדולה יותר, כך גדלה האנרגיה הקינטית המוענקת מההתנגשויות לאלקטרונים הבורחים. איינשטיין גם טען שעוצמת האור אינה תלויה במשרעת הגל הנכנס, אלא מתאימה למספר הפוטונים הפוגעים במשטח בפרק זמן נתון. מספר האלקטרונים הנפלטים עולה עם הבהירות. ככל שמספר הפוטונים הנכנסים גדול יותר, כך גדל הסיכוי שהם יתנגשו בחלק מהאלקטרונים. 

 האפקט הפוטואלקטרי מרמז מאוד על התנהגות החלקיקים של האור. איינשטיין זכה בפרס נובל לפיזיקה בשנת 1921 על ההסבר שלו על האפקט הפוטואלקטרי. למרות שניתן להסביר תופעות אור רבות במונחים של גלים או חלקיקים, תופעות מסוימות, כמו דפוסי ההפרעות שהתקבלו כאשר האור עבר דרך חריץ כפול, היו מנוגדות לחלוטין לראיית חלקיקים של האור, בעוד שתופעות אחרות, כגון אפקט הפוטואלקטרי, היו מנוגדים לחלוטין לנוף של גלי אור. איכשהו, ברמה יסודית עמוקה שעדיין לא מובנת במלואה, האור גם דמוי גל וגם חלקיקי. זה ידוע בתור דואליות גל חלקיקי.

 טקסט זה מותאם מ <a href="https://openstax.org/books/chemistry-2e/pages/6-1-electromagnetic-energy">Openstax, Chemistry 2e, Section 6.1: Electromagnetic Energy</a>.</div>

כאשר אור באורך גל מסוים פוגע במשטח מתכת, אלקטרונים נפלטים. זה נקרא אפקט פוטואלקטרי. התדר המינימלי של ...

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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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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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