What is photoelectric effect class 12 – Modern Physics – eSaral

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## What is Photoelectric effect?

The phenomenon of emission of electrons from the surface of the metal when the light of suitable frequency falls on it is called the photoelectric effect.

1. The ejected electrons are called photoelectrons
2. The current produced due to emitted electrons is called photocurrent.
3. The photoelectric effect proves the quantum nature of radiation.
4. The classical electromagnetic theory fails to explain photoelectric effect.
5. Einstein explained the photoelectric effect using quantum nature of radiation.
6. Hallwach is credited with discovery of the photoelectric effect.

## Experimental study of photoelectric effect

### Effect of intensity of incident radiation

1. The number of incident photons per second on a metal plate is called intensity of incident radiation.
2. For a fixed incident frequency, the saturation photocurrent is directly proportional to the intensity of incident radiation e.g. when intensity of radiation is doubled at constant frequency saturation photocurrent is also doubled.
3. Saturation current: When all photo electrons produced reach anode the photocurrent becomes maximum and is independent of applied potential difference. This current is called saturation or maximum current.

### Effect of potential

1. When polarity of electrodes is reversed with commutator the current is reduced but does not become zero. This shows that emitted photoelectrons have kinetic energy.
2. The negative potential of the anode at which the photo current becomes zero is called stopping potential $\left(\mathrm{V}_{\mathrm{s}}\right)$. At this potential the electrons with maximum kinetic energy are stopped from reaching the anode.
3. No photo current is produced even on increasing the intensity of incident radiation when the anode is at stopping potential. Thus, stopping potential is independent of the intensity of incident radiation.
4. The stopping potential is a measure of maximum kinetic energy of photo electrons. $E_{\max }$ $=\mathrm{eV}_{\mathrm{s}}$

### Effect of frequency of incident radiation

1. Threshold frequency the minimum frequency of incident radiation that can eject photo electrons from a metal is known as threshold frequency $\left(v_{0}\right)$.
2. At stopping potential if the frequency of incident radiation is increased then current starts flowing again. This can be made zero by increasing stopping potential.
3. Thus, the maximum kinetic energy of photoelectron or stopping potential increases with an increase in the frequency of incident radiation. The maximum kinetic energy of photoelectron increases linearly with increase in the frequency of incident light.

### Effect of material of cathode

The stopping potential, work function, and threshold frequency depend on the nature of material of cathode. Work function : The minimum energy required for emission of electrons from metal is called work function work function $\phi=\mathrm{hv}_{0}$ where $v_{0}$ is threshold frequency.

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