Photoelectric Effect: Class 12 Physics Notes, Definition, Working Principle, Formula & Real-Life Applications

Let us first understand what photoelectric effect is in detail. The phenomenon known as the photoelectric effect occurs when electromagnetic radiation of an appropriate wavelength strikes a metallic surface, causing electrons to be released.

1. Photoelectron: The term "photoelectron" refers to the electron released in a photoelectric effect.
2. Photoelectric current: A current is referred to as a photoelectric current if it flows via a circuit in a photoelectric effect.
3. Work function: The work function is the least amount of energy needed to liberate an electron from a metal. It is represented by ϕ or W and is constant for a metal. For Caesium, it is the bare minimum. For alkali metals, it is comparatively lower.

Only metal and light are needed to create the photoelectric effect, but the circuit is finished for viewing. A configuration used to investigate the photoelectric effect is depicted in the figure.

In this case, plate (1) is referred to as the emitter or cathode, and plate (2) is referred to as the collector or anode.

1.4 Saturation current: The highest amount of photoelectric current occurs when 100% of the photoelectron emission from the cathode reaches the anode. This current is known as the saturation current.

1.5 Stopping potential: The stopping potential is the lowest value of the anode's negative potential relative to the cathode for which the current is zero. Another name for this is cutoff voltage. The intensity has no bearing on this voltage.

1.6 Retarding potential: Retarding potential is the negative potential of the anode relative to the cathode that is less than stopping potential.

“It was found that certain metals like zinc, cadmium, magnesium, etc., responded only to ultraviolet light, having short wavelength, to cause electron emission from the surface. However, some alkali metals such as lithium, sodium, potassium, caesium and rubidium were sensitive even to visible light. All these photosensitive substances emit electrons when they are illuminated by light. After the discovery of electrons, these electrons were termed as photoelectrons. The phenomenon is called photoelectric effect.”

So, when light hits certain metals, some of them start kicking out tiny particles—those are electrons. But here’s the thing: not every metal reacts the same. A few only respond if the light has a lot of energy, like ultraviolet. Others, like sodium or potassium, don’t need that much. Even visible light works.

We call those metals photosensitive substance. Basically, they react when light falls on them. The electrons they spit out? Scientists later called them photoelectrons. And the name for the whole thing, where light knocks electrons loose? That’s the photoelectric effect. Simple, but it ended up being kind of a big deal in physics.

Let us now discuss the Einstein photoelectric effect equation in this section. As per Einstein, light is a wave that interacts with matters as quantum/packet of energy. The photon is quantum of radiation and the equation is known as the Einstein photoelectric effect equation. Einstein derived his equation and subsequently expanded Planck's notion.

According to Einstein, light consists of discrete energy packets called photons. When a photon hits the surface of a metal, it transfers its energy to an electron. If this energy is large enough to overcome the metal’s work function (the minimum energy needed to release an electron), the electron is emitted.

Einstein expressed this with the equation:

$h\nu =\varphi +K_{\text{max}}$

As per Einstein's photoelectric effect equation, the arriving photon has an energy of hvh\nuhν.

$\begin{array}{ll}h& \text{Planck’s constant}\\ \nu \text{(nu)}& \text{the frequency of the incident light}\\ \phi \text{(phi)}& \text{the work function of the metal}\\ K_{\max }& \text{the maximum kinetic energy of the emitted electron}\end{array}$

No matter how strong the light is, no electron is released if the photon's energy is less than the work function.

The notion that light acts as both a wave and a particle (photon) was validated by Einstein's equation. He was awarded the Nobel Prize in Physics in 1921 for this explanation, which also served as the basis for quantum mechanics.

Dual nature of radiation and matter is also an important chapter for JEE Mains exams and NEET exam. 

The following examples illustrate how the photoelectric effect is used in real-world situations:

• Automatic streetlights use photoelectric or photocell cells. The photoelectric effect is used to detect changes in light intensity in the evening, causing the lights to automatically turn on.
• When sunlight strikes semiconductors in solar panels, electrons are liberated, producing electricity. The Einstein photoelectric effect equation is directly applied to renewable energy in this way.
• To assist photographers gain the proper exposure, light meters in cameras measure the amount of incident light using the photoelectric principle.
• Infrared sensors and TV remote controls both use the photoelectric effect, in which light-based impulses cause particular electronic reactions in the equipment.

It is advisable to practice the NCERT solutions of the chapter to perform well in the examinations.