what is photoelectric effect ?

Photoelectric effect Explained 

The phenomenon is discovered by Heinrich Hertz . who was a German physicist.

When a beam of radiation of a certain frequency falls on the metal surface, the electrons are emitted from the metal surface

 It is a phenomenon of instant ejection of the electron from a metal when the light of a particular frequency/wavelength is incident on the metal surface

Parameter

1.    Frequency of light (color of light)

2.    Intensity of light (brightness)

·      Frequency effect

When we use low frequency number electron ejected from the metal surface when we increase the frequency of light slowly after a certain value of frequency the electrons start ejecting from the surface of the metal and when be further increase the velocity of the electron the velocity of ejected electron increases.

The particular value of frequency, when the electron start ejecting from the metal surface,

is called the threshold frequency it is denoted by ( ν₀ ) letter nu with subscript zero

                                                               Or

The minimum amount of frequency required to remove the electron from the surface of the metal is called threshold frequency

                                  λ=c/ ν₀

   λ is threshold wavelength

·      Intensity

we choose the frequency above the threshold frequency .we absorbed that when we increase the intensity of the light the number of electrons ejected increase  and with decreasing intensity the number of electron ejected decreases there is one effect on the velocity of the ejected electron.

·      Summary

·       For photoelectric emission we required a minimum amount of frequency/wavelength is required is called threshold frequency or threshold wavelength.

·       When we increase the frequency of light the speed of ejected electron increase that is the kinetic energy of ejected electron increases (Photocurrent remain same)

·       When we increase the intensity of the light the number of electron ejected increase that is photocurrent increases (Kinetic Energy remain same of the ejected electron)

Einstein’s Theory of Photoelectric Effect using Quantum Theory

Einstein gives a successful explanation of Photoelectric effect.

 Einstein made three simple Postulate

·       Light is made up of photons with energy (E = hν )    (where h is plank’s constant and ν is the frequency of light radiation)

·       Each photon can eject only one electron

·       Give a simple governing equation of the photoelectric effect

                                                 hν = hv_o +1/2 mv²

where

·       hν is the energy of the Photon

·       hv_o  is called Work function it is also denoted by ɸ ( v_o  is the threshold frequency)

·1/2 mv² is the kinetic energy of the ejected electron

Theory

Einstein use planck’s quantum theory  to give an explanation of the photoelectric effect. According to the Planck’s theory, the light is made up of the discrete packets of the energy called a Photon. Each photon has energy equal to hν (where h is plank’s constant and ν is the frequency of light radiation) when photon of frequency ν strike with the electron  in a metal its transfer its energy hν to the electron

The energy is use in two ways

·       some part of the energy is used to overcome the threshold energy barrier of the metal ( is called work function )

·       and the remaining energy is converted into the kinetic energy of the ejected electron

                                             hν = hv_o + 1/2mv²  

                                                  or 

                                       hν = hv_o + K.E.

that is                              K.E = hν- hv_o

this equation show that The kinetic energy of the electron depends upon the frequency of the incident light and not on its intensity i.e. when the intensity of light is increased the number of photons striking the metal surface per unit time increases. thus more number of electrons will be emitted, however, the kinetic energy remains unaffected.

Stopping Potential

Is defined as the value of the reverse voltage  applied where the velocity of electron becomes zero is called stopping potential v₀