The emission of electrons with help of electromagnetic radiations 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.
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.
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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 being quantized. 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:
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:
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 a number of devices and processes. Some of which are:
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 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 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.
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 are indispensable for the study of quantum physical sciences. This is required for constructing various devices and various phenomena to occur.
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. Hens most of the photo-devices are coated with these metals.