photoelectric effect photoelectrons

Why Photoelectric Effect Will Make You Interested in Physics


Introduction

Photoelectrons are electrons emitted from a metal surface. Photoelectric Effect is formed by the movement of these electrons in a circuit. A metal has numerous free electrons that float about the metal’s body. However, these electrons are not free to exit the metal’s surface. Whenever they try to escape the metal, it pulls them back.

German physicist Heinrich Hertz
German physicist Heinrich Hertz

Definition of Photoelectric Effect

When an appropriate wavelength of radiation is impacted on the surfaces of particular metals, electrons are freed from the metal’s surface. This effect is known as the photoelectric effect, which was discovered by German physicist Heinrich Hertz in 1887.

Photoelectrons are electrons emitted from a metal surface. Photoelectric current is formed by the movement of these electrons in a circuit. Different metals react photoelectrically to different wavelengths of energy. To get an electron out of a metal, a minimum quantity of energy corresponding to the work function must be applied.


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Work Functions of Different Metals Undergoing Photoelectric Effect

Work function for the metal Caesium is 1.9eV, for Aluminium it is 4.08eV, for Potassium 2.28eV, for Zinc 4.31, for Sodium 2.3eV, for Iron 4.50eV, for Lithium 2.5eV, for Calcium 3.2eV, for Silver 4.70eV, for Lead 4.13eV and for Platinum 6.35eV.


Experimental Study of Photoelectric Effect

The photoelectric effect can be studied experimentally. A photosensitive anode and cathode are sealed in an evacuated glass tube bulb. The cathode is attached to the battery’s negative terminal. The anode is connected to the positive terminal through a microammeter, that detects the photocurrent.

The glass bulb features a side tube featuring a quartz window that allows radiation to enter. When radiation strikes its cathode, photoelectrons are released, which go to the anode. This may be used to investigate the relationship between photoelectric current and incident radiation frequency, the potential difference between cathode and anode, the type of photo emissive surface, and incident radiation intensity.


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Photoelectric Effect Facts

We may conclude from the experiment that photoemission is immediate. The inexplicable conclusion is that there is no time lag between the onset of irradiation and the commencement of the photocurrent. Measurements have revealed that if there is a temporal lag, it is lower than 10-8s.

For every photosensitive surface, there is a certain frequency of the incident radiation below which there is no photoemission. This minimum frequency is called threshold frequency ν0.

The corresponding wavelength is called threshold wavelength λ0. If the frequency of the incident radiation is less than the threshold value (ν < ν0) electrons are not liberated no matter how long the radiation falls on the surface or how strong its intensity is.

The electrical current is dependent on the intensity of radiation at a specific frequency higher than the threshold value. The quantity of photoelectrons grows as light intensity increases.


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Intensity vs Frequency in Photoelectric Effect

Raising the intensity of light with a set frequency below the threshold frequency will not generate a photoelectric effect. With the increasing frequency of incoming light, the maximal kinetic energy of the released photoelectron rises.

It is independent of input radiation intensity and is measured by adding a retarding voltage and progressively raising it till the strongest energetic electrons quit and the photocurrent reaches zero. The minimal negative voltage necessary to terminate electron flow to the anode (photoelectric current in the circuit becomes zero) is known as retarding potential or stopping potential.

If the frequency of the radiant energy is greater than the threshold value of the photo emitter, the number of photoelectrons released in a photoelectric activity is exactly proportional to its intensity.


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How Einstein Explained Photoelectric Effect

In a photoelectric process, the maximal kinetic energy of the released electrons is exactly dependent on the frequency of the input light. On the basis of the quantum theory of radiation, Einstein provided a reasonable explanation for the photoelectric phenomenon by assigning particle nature to electromagnetic waves.

Maxwell’s classical theory, in other words, is successful in explaining the movement of light through space over large time intervals, but a new theory may be required to describe brief exchanges of light and matter. He claimed that light energy is not spread equally throughout the classical wavefront, but rather is contained in discrete regions (or ‘bundles’) termed quanta, each of which contains energy hv.

According to Einstein’s model, a light quantum is so confined that it gives all of its energy = hν, straight to a single electron inside the metal. Therefore, according to Einstein, the maximum kinetic energy for an emitted electron is Kmax = hν – ϕ.


photoelectric effect stopping potential voltage Zn

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Work Function in Photoelectric Effect

Where ϕ is the work function of the metal, which corresponds to the minimum energy with which an electron is bound in the metal. It is a constant for a given photoelectric metal.

Einstein’s equation mentioned above satisfactorily explains the experimental observations on photoelectric emission.

For a fixed light frequency ν, an increase in light intensity means more photons and more photoelectrons per second, although Kmax, remains unchanged. Light of threshold frequency ν0 which has just enough to knock an electron out of the metal surface, causes the electron to be released with zero kinetic energy.

Thus, the variation in threshold frequency for different metals is produced by the variation in work function. Light with ν < ν0 has insufficient energy to free electrons. Consequently, the photocurrent is zero for ν < ν0. Kmax should vary linearly with frequency and the slope of the Kmax vs ν graph should yield the universal constant h.

The photoelectric effect is explained on the basis that, the incident light energy appears in small energy packets and there is a one-to-one interaction between photons and electrons. Thus, it signifies the particle nature of light waves.


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