The emission of electrons from a metal surface on the
incidence of electromagnetic radiations is called

__“Photoelectric Effect”.__
The basic principle behind this effect is that the radiations
transfer the energy to electrons inside the metal and if this energy is
sufficient then the electrons are ejected from the surface. According to
Maxwell’s Theory the ejection of electrons should depend on intensity of
radiation that is if electrons are not being ejected, then on increasing the intensity
they can be ejected and so on. The following observations are made: i.e. the
energy of the electrons should have been proportional to the intensity of the
light, not the frequency.

1. No
electron is ejected if the incident frequency, ν, is less than certain minimum value ν

^{0}.
2. When ν < ν

^{0}then even on increasing the intensity here is no ejection of electrons.
3. When ν > ν

^{0}, howsoever low the intensity is, ejection is observed.
4. When ν > ν

^{0}, on increasing the intensity the number of photoelectrons keep on increasing.*These findings again contradict Maxwell’s theory.*

##
__Explanation of
Photoelectric Effect:__

__Explanation of Photoelectric Effect:__

As one electron absorbs one quantum that is why if the energy
of one quantum is insufficient for the ejection of electrons then there is no
use of increasing the number by increasing the intensity. To make ejection
possible the energy per Quantum should be increased by increasing frequency.
And for every metal there is a minimum value of frequency at which ejection
starts which is called “Threshold Frequency” represented by “ν

^{0}”.
So the following
observations were made:

1. When ν < ν

^{0}then energy of one quantum is insufficient, ejection does not take place
2. When ν = ν

^{0}then ejection just takes place and electrons don’t have any kinetic energy.
3. When ν > ν

^{0}then every ejected electron possesses kinetic energy that is the excess energy of a quantum gets converted into kinetic energy of the electron. The Threshold energy required for emission is called “Work Function” (f) that is “h ν^{0}”.
Thus by energy balance:

Energy of
one quantum = Threshold Energy + Kinetic Energy

*h*

*ν*

*= h*

*ν*

^{0}

*+ (1/2)m*_{e}v2

4. When ν > ν

^{0}, then on increasing the intensity the number of quanta incident increases thereby increasing the number of photoelectrons ejected.
The Kinetic energy of electron varies linearly with the
incident frequency as shown in the curve. Einstein gave this equation of
photoelectric effect and he concluded that Light has particle like properties
as well.

We can conclude that according to Maxwell, Light is a wave
but according to modern theory it may also have particle like properties, as it
contains photons each with discrete amount of energy. Therefore, Light has both
the properties: Wave and Particle.