Photoelectric Threshold Frequency

Work Function and Threshold Frequency

The photoelectric effect is a phenomenon in which light causes electrons to be released from a metal's surface. Photoelectrons are the electrons that are expelled. It's worth noting that the frequency of the light incident on the metal's surface affects the emission of photoelectrons and the kinetic energy of the expelled photoelectrons. Photoemission is the term used to describe the process by which photoelectrons are emitted from the metal's surface owing to the action of light.

The photoelectric effect happens when electrons at the metal's surface absorb energy from incoming light and utilise it to overcome the attractive forces that bind them to the nuclei of the metal. In this article, students will get to learn about photoelectric threshold frequency in detail. The material is revised and provided by Vedantu’s subject matter expert with an in-depth concept and easy explanation. Students preparing for JEE, NEET and other competitive exams can get an upper hand if they study the topic from Vedantu as Vedantu covers the topic very concisely.


Work Function and Threshold Frequency

The emission of electrons with help of electromagnetic radiation like light from the surface of a photoelectric material is called the photoelectric effect.  The electrons emitted are known as photoelectrons. The photoelectric threshold frequency is the minimum frequency of the electromagnetic rays which when strikes a surface, causes a photoelectric effect. The photoelectric effect is a subject of study for quantum chemistry, condensed matter physics, for the behavioural study of various properties of solids, atoms, and molecules. The concept of emission of electrons from metal surfaces when electromagnetic waves, typically of short wavelength like the visible or ultraviolet rays strike the surface is known as photoconductive, photoelectrochemical effect since it is initiated by light energy. 

 

Photons

A photon can be defined as a quantum of light that carries energy proportional to radiation frequency but has zero rest mass, and moves at the speed of light in the vacuum. Photons belong to the class of Boson and are elementary particles. The energy of light is considered to be quantized and hence, these small packets or quanta of energy are known as photons. Max Planck while studying Black Body Radiation discovered this fact. 

 

Work Function and Threshold Frequency Formula

The first theory of the photoelectric effect was put forward by Einstein by using Max Planck’s theory of light energy. It was considered that each packet of light energy or photons carried energy hv where h was a proportionality constant known as the Planck constant and v was the frequency of the electromagnetic waves of light. Kmax is the maximum amount of kinetic energy delivered to the atoms before they leave their atomic bonding. To explain threshold frequency we can write the equation for photoelectric effect as: 

Kmax=hv-W

Here W is the work function of the metal. It is the minimum energy that needs to be supplied to the metal body for the emission of photoelectrons. Now W can be written as: 

W= hvo

Here vo is the photoelectric threshold frequency of the electromagnetic rays. 

 

Steps for Photoemission from Metal Surfaces

The process of photoemission is a multistep process. These are:

  • The first step involves the striking of the electromagnetic radiation on the metal surface which causes the excitation of electrons. 

  • The second step involves overcoming the energy barrier. The electrons impart the energy given to them by the photons. Or it can be said that the energy of photons is transmitted to electrons.

  • The valence electrons of the metal which are bound loosely to the nucleus start leaving the metal body with the help of the excess energy provided after utilising the work function of the metal.

 

Uses of Photoelectric Effect Threshold Frequency 

The concepts threshold energy in photoelectric effect and threshold frequency find their application in many devices and processes. Some of which are:

Photomultipliers: 

These are extremely light-sensitive vacuum tubes that have quoted photocathodes with caesium, rubidium and antimony for providing low work function as these metals have extremely low work function. This is so that when illuminated by very low levels of light the photocathodes start releasing electrons and photocurrent can be detected.

Photoelectron Spectroscopy: 

Photoelectron spectroscopy measurements are done in a high vacuum environment to prevent electrons from being scattered by gas molecules present in the air. In this process monochromatic X-ray or UV rays of known frequency and kinetic energy are used to determine experimentally the composition of area samples.

Night Vision Devices: 

Photons when strike alkali metal or semiconductor material like gallium arsenide in an image intensifier tube, causes the ejection of photoelectrons because of the photoelectric effect. This is accelerated by an electrostatic field where they strike a phosphor-coated screen thus converting electrons back into photons. Signals are generated and intensified due to the acceleration of electrons or increasing the number of electrons from the secondary emission. This concept is used in night vision devices.

Image Sensors: 

Television in the early days had video camera tubes that used the photoelectric effect to transform an electronic signal into an optical image. However, currently, the mechanism of television working has changed.

As seen above, the concept of photoelectric emission, work function and photoelectric threshold frequency is indispensable for the study of quantum physical sciences. This is required for constructing various devices and various phenomena to occur.

 

Discovery Of Photoelectric Effect

Wilhelm Ludwig Franz Hallwachs initially proposed the photoelectric effect in 1887, while Heinrich Rudolf Hertz carried out the experimental proof. They discovered that when a surface is subjected to higher-frequency electromagnetic radiation, the energy is absorbed and electrons are released. The photoelectric effect is now understood to be a phenomenon in which a substance absorbs electromagnetic radiation and releases electrically charged particles.

To be more specific, the photoelectric effect causes electrons to be expelled when light is incident on a metal's surface. A photoelectron is an electron that is emitted as a result of the photoelectric effect and is symbolised by the letter e–. Photoelectric current is the current generated as a result of the expelled electrons.

 

Did You Know?

The lower the work function of a metal, the easier it is for us to achieve the process of photoelectric emission and lower the photo-energy required for it to emit electrons. Caesium, lithium, antimony are perfect metals to obtain photoelectric emission phenomena because of their low work function. Most of the photo devices are coated with these metals.

FAQs (Frequently Asked Questions)

1. Define the Term Threshold Frequency in the Context of Photoelectric Emission. What is the Formula of Threshold Frequency?

Threshold frequency in photoelectric effect is the minimum frequency of electromagnetic radiation which is required for the phenomenon of photoelectric emission to occur from a metal surface. It is a characteristic of electromagnetic radiation (light in case of photoelectric emission) and not metal. 


The formula of threshold frequency is W= hv0. Here v0 is the photoelectric threshold frequency of the electromagnetic light rays, W is the work function of the metal body. There is no term like the threshold frequency of metals because threshold frequency is the characteristics of the electromagnetic radiations and not the metal surface. 

2. Explain the Process of Photoelectric Emission.

Photoelectric emission can be defined as a process in which on striking of photoelectric energy or light on a metal body, the photons or packets of light energy induce its energy on conduction electrons of metals which results in them leaving the metal surface. Every metal has its own work function which is the minimum amount of energy required for photoelectric emission to start the process of photoelectric emission. The energy imparted by two electrons or the metal body on the striking of photons is equal to the sum of the work function of the metal and the kinetic energy of the electrons during the emission.

3. What are the applications of the Photoelectric Effect?

The Photoelectric effect is subjected to various applications in day-to-day life as well as major scientific applications. Some of the applications of the photoelectric effect are listed below:

  • Lighting sensors, such as those used in smartphones, allow for automated screen brightness change in response to ambient light. This is because the quantity currently created by the photoelectric effect is proportional to the amount of light that strikes the sensor.

  • Solar Panels use it to create power. Metal combinations in these panels allow power to be generated from a wide variety of wavelengths.

  • A photoelectric semiconductor is put in front of a UV or IR LED in motion and position sensors. Light is switched off when an item is placed between the LED and the sensor, and the electrical circuit recognises a change in potential difference.

  • Because digital cameras feature photoelectric sensors that respond to different colours of light, they can detect and record light.

  • X-Ray Photoelectron Spectroscopy (XPS): This approach involves irradiating a surface with x-rays and measuring the kinetic energy of the electrons that are released.

4. What is the photoelectric effect and why is it important?

Our knowledge of the quantum nature of light and electrons has grown as a result of research into the photoelectric effect. It has also affected the development of the wave-particle duality notion. The photoelectric effect is also commonly used to look at the energy levels of electrons in substances. The work function is the metal surface's threshold energy for ejecting electrons, which is commonly expressed in Joules. If the photon's energy exceeds the threshold, the expelled electron will have surplus energy, also known as kinetic energy, when it is released. As a result, the higher the photon's energy is compared to the threshold energy, the more kinetic energy the electron has when it is expelled.

5. What are the key characteristics of the photoelectric effect?

 It has three characteristics: 

(1) it is immediate

(2) it happens only when the radiation is over a cut-off frequency and 

(3) photoelectrons' kinetic energy at the surface is independent of the intensity of the radiation. Classical theory cannot explain the photoelectric effect.

Below is an example of Sodium ions omitted out from its surface after a certain frequency of light or threshold frequency of light made fall on the surface of the sodium metal:

  • The electrons were quickly ejected.

  • Increasing the light intensity increased the number of photoelectrons but did not enhance their maximal kinetic energy.

  • When Na is the metal, no amount of red light, no matter how intense, can trigger electron ejection.

  • Although only a few electrons are ejected by weak violet light, their maximal kinetic energies are higher than those ejected by the bright light of longer wavelengths.

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