Photo electric effect

The Photo Electric Effects

By :- Nikhil Merothiya

We know that the light has dual nature.

The Phenomena of interference & diffraction etc. can be explained on the basis of wave theory of light.

Whereas the phenomena of Photoelectric Effect , compton effect can be explained on the basis of particle theory of light.

The Photo Electric Effect

The ejection of electrons from a metallic surface when the light of suitable frequency is allowed to fall on the surface.

The Photo Electric Effect

This suitable frequency is called Threshold Frequency and the corresponding wavelength is called threshold wavelength.

Work function: The work function is the energy required to remove an electron from the highest filled level in the Fermi distribution of a solid.

Typical Experimental Setup

• Incident light triggers the emission of (photo)electrons from the cathode.• Some of them travel toward the collector (anode) with an initial kinetic energy.• The applied voltage V either accelerates (if positive) or decelerates (if negative) the incoming electrons.• The intensity I of the current measured by the ammeter as a function of the applied voltage V is a measurement of the photoelectron properties, and therefore a measurement of the properties of the photoelectric effect.

How to understand the Photoelectric Effect?

Think about hitting a ball into outer space.

If you don't hit it hard enough, it will just come back down. No matter how many times you hit it.If superman hit it, he could get it into space.

Similarly, no matter how many photons strike the metal, if none of them has sufficient energy to eject an electron from a metal atom, you won't get a current.If the energy the taken up by the electron is sufficient to allow it to be released from the metal atom, you will get a current.

Relation between energy of photon & emitted electron’s

maximum kinetic energy

E

Km

W

Km :- maximum kinetic energy of emitted electron

W :- work function

Relation between current & applied potential difference

V

V :- potential difference

Vs :- stopping potentialʋ :- frequency (constant)

Relation between current & frequency

Vs1 Vs2 Vs3

Ʋ1

Ʋ2 Ʋ3

The stopping potential depends on the frequency:-Higher frequencies generates higher energy electrons.

Ʋ1 > Ʋ2 > Ʋ3

Laws of Photoelectric effect

Photoelectric effect is directly proportional to intensity. If the frequency of the incident light is less than the threshold frequency then no electron ejected, no matter what the intensity.

The maximum kinetic energy of the electrons depend on the frequency of the incident light.

The electrons were emitted immediately - no time lag.

Einstein’s equation

Km = E - W

Km:- maximum kinetic energy

E :- energy of photon’s

W :- work function of metal

Km

WE

Einstein’s equation

Use of Photoelectric effect

Photo cell

THANKING YOU