Photoelectric Effect

Photoelectric effect 

When the atoms of a metal absorb electromagnetic radiation, they eject electrons causing 

a flow of current in a circuit. This phenomenon is called photoelectric effect. It was observed that blue light incident on Potassium could eject photoelectrons but red light failed to eject any electron from the same metal. The energy of the ejected electrons did not depend on the intensity of the incoming light, but it depended upon its frequency.


When low-frequency light of weak intensity was incident upon a metal, a few low energy electrons were ejected. Increasing the intensity of light, increased the number of ejected electrons but these electrons had the same low energy. In order to eject high energy electrons, one must illuminate the metal with high-frequency light. 

Einstein used Planck's energy quanta (which determines the energy of the photons based upon its frequency) to explain the photoelectric effect. These are quanta of light energy with particle nature. The electromagnetic radiation of a given frequency can only transfer energy to matter in integer multiples of an energy quantum hν. Only photons of a high enough frequency (above a certain threshold value) could knock out an electron. Low- frequency light can eject only low-energy electrons because each electron is excited by the absorption of a single photon. Increasing the intensity of the low-frequency light (increasing the number of photons) only increases the number of excited electrons, not their energy. But by increasing the frequency of the light, and thus increasing the energy of the photons, can one eject electrons with higher energy. The higher the frequency of a photon, the higher will be the kinetic energy of the emitted electron,


The energy of ejected electrons should increase linearly with frequency and the gradient of this plot should be the Planck's constant h (= 6.626×10−34 J–seconds). This idea of Einstein was confirmed several years later by Millikan, whose experimental results were in perfect accord with Einstein's predictions. Einstein got Nobel Prize for his revolutionary suggestion of quantized light. Still later, Einstein's "light quanta" got approved to be called as photons, when their existence was demonstrated through photon anti-bunching effect. The propagation of electromagnetic radiation follows the laws of linear wave equations, but its emission or absorption happens only as discrete elements. Thus it acts as a wave and also as a particle simultaneously.


After observing the wave-like properties of small particles such as photons and electrons,

experiments were done to explore the wave nature of larger objects such as neutrons and protons and even the atoms and molecules. The results indicate that these larger particles also act like waves. A wave is basically a group of particles which moves in a particular form of motion, i.e. to and fro. If we break that flow by an object it will convert into radiants. The photoelectric effect is the most direct and convincing evidence of the existence of photons and the 'corpuscular' nature of light and electromagnetic radiation. It is undeniable evidence of the quantization of the electromagnetic field. For explaining the photoelectric effect and modifying the classical field equations of Maxwell, Einstein was accorded the Nobel Prize.