Photoelectric effect

Photoelectric effect: A history

In 1887, Hertz noticed that electrons are emitted from a metal surface when electromagnetic radiation falls. In 1888, Hallwachs showed experimentally that electrons are emitted from the Zinc plate when ultraviolet rays fall on the plate.

This phenomenon of emission of electrons from a metallic surface, when illuminated by the light of appropriate wavelength or frequency, is called the photoelectric effect. The electrons radiated in this process are called photoelectrons, and the current produced in the circuit is called photoelectric current.

Photoelectric effect: Definition

The photoelectric effect is a physical phenomenon that occurs when a metal surface is struck by light of a specific frequency. Heinrich Hertz discovered this phenomenon back in 1887, Lenard confirmed it in 1902. This effect has been found useful in electronic devices specialised for detecting light and precisely timed emissions of electrons. 

Principle of the photoelectric effect

In 1887, Hertz noticed that electrons are emitted from a metal surface when electromagnetic radiation falls. In 1888, Hallwachs showed experimentally that electrons are emitted from the zinc plate when ultraviolet rays fall on the plate.

The electrons radiated in this process are called photoelectrons, and the current produced in the circuit is called photoelectric current.

The photoelectric effect, in general, is a phenomenon exhibited by all substances when illuminated by radiation of a suitable wavelength.

Particle nature of light: The photon

Einstein gave his theory of the photoelectric equation which means that electrons don’t absorb radiation continuously. Still, radiation energy is made up of discrete units called quanta of energy of radiation. He proved his theory and led to the following equation, commonly known as Einstein’s photoelectric equation. 

KE = hv – hv0 where KE is the kinetic energy of the photoelectron, h is Planck’s constant, v is the frequency of the photon, and v0 is the threshold frequency of material. The particle nature provides a valid reason for the behaviour of matter. It proves that particles of any matter are always moving, explaining solid, liquid, and gas behaviour and properties. 

Threshold Frequency

Hallwachs and Lenard also observed that when the frequency of the incident UV light was smaller than a specified minimum value, known as the threshold frequency, then no electrons were emitted at all. This minimum frequency was dependent on the nature of the material of the emitter plate. Metals like zinc, cadmium, magnesium, etc., were found to respond only to UV light with short wavelengths to cause electron emission from the surface. However, other alkali metals such as lithium, sodium, potassium, cesium, and rubidium were sensitive even to visible light. The current produced due to the photoelectric effect is known as photoelectric current.

Properties of photoelectric current

The experimental study of the photoelectric effect gave us the following observations.

Effect of intensity of light on photocurrent

The photoelectric current is directly proportional to the intensity of the incident light for a given photosensitive material and the frequency of incident radiation above the threshold frequency.

Effect of potential on photocurrent

The stopping potential is independent of light intensity. In contrast, the saturation current is proportional to the intensity of incident radiation for a given photosensitive material and the frequency of incident radiation.

Effect of frequency of incident radiation on stopping potential

KMax = eVo

1. Einstein’s photoelectric equation

Albert Einstein in 1905 proposed that radiation energy is built up of discrete units called quanta of energy radiation. Each quantum of radiation has energy h, where h is the Planck’s constant and is the frequency of light radiation. A quantum of energy is absorbed by the electron and if it exceeds the minimum energy needed for the electron (Work Function 0 ) .

 KE = hv – hv0 

where KE is the kinetic energy of the photoelectron, h is Planck’s constant, v is the frequency of the photon and v0 is the threshold frequency of material. 

Electrons that are more tightly bound emerge with less kinetic energy than the maximum value. The photoelectric process is instantaneous regardless of the light intensity. Since the basic elementary processes, the same low intensity does not mean a delay in emission. Only the number of electrons that can participate in the absorption of light quantum by a single electron is determined by the intensity of the light radiation and, therefore, the photoelectric current.

eVO =   h–0; for >0

or VO =  (he)–0e

The VO vs is a straight line with slope = (he)

Particle nature of matter

The particle nature provides a valid reason for the behaviour of matter. It proves that particles of matter are constantly moving, explaining solid, liquid, and gas behaviour and properties. 

• The photoelectric effect gave evidence that when light interacts with a matter, it behaves as a packet of energy that is hv. Each photon has its energy E = hv and momentum p = hv/c, where c is the speed of light in a vacuum.
• Photons are not dependent on the intensity of the radiation used to strike the surface.
• Photos are not deflected by magnetic and electric fields, which are neutral.
• The total energy is conserved in photon-electron collisions.
• The particle nature is depicted in experiments such as the photoelectric and coupon effects. 

Einstein described the photoelectric effect by referring to light as having particles, as it does in nature.

Photoelectric Work Function

The photoelectric effect removes the electron from an atom of a photosensitive material that is the most loosely linked. The surface material’s photoelectric work function (Ø0) is defined as the smallest amount of energy required to eject an electron from a specific surface when the surface material is a semiconductor. The work function of a metal surface is one of its distinguishing characteristics.

The work function can be expressed mathematically as

Ø0 = hv0

Conclusion

In this article, we learned about the photoelectric effect discovered by Albert Einstein, for which he was awarded the Nobel Prize in Physics in 1921, and its principles. The photoelectric effect is a physical phenomenon that occurs when a metal surface is struck by light of a specific frequency. Heinrich Hertz discovered this phenomenon back in 1887, Lenard confirmed it in 1902. We also learned about the threshold frequency, work function, photoelectric current, and properties. The minimum frequency required for electron emission is known as the threshold frequency. We also learned about the particle nature of light and the photon. The light quantum is associated with a particle and has a definite value of energy and momentum, and is known as the Photon. This chapter is significant, and to better understand it, other topics such as the wave nature of light and De Broglie’s equation are suggested to the user.