Photoelectric
effect is a phenomenon whereby electrons are ejected from a metal surface when
an incident light falls on it. The
removal of electron from the metallic surface can also be called as
photoemission or photoelectric emission. the electrons ejected are called photoelectrons and the substance that undergo this
phenomenon is called photo-emissive
surface. The electrons ejected are same like other electrons with same
mass, charge and spin.
For an
electron to be ejected from a metal surface , the incident light must have a
frequency higher than the threshold frequency, tough different material have
different frequencies but most metal have their frequency in the ultraviolet
region.
An electron
escaping from a metal surface has the maximum velocity while an electron
produced deep inside the metal has lesser velocity, it might have lose some of
it energy in coming out to the surface.
The voltage
needed to keep the electron within the metal is called the stopping potential. When
this potential is not overcome, the electron will not be ejected and current will
not flow through the photoelectric
circuit.
The maximum
kinetic energy of the electrons depend on the stopping potentials, it depend on
the frequency of the incident photon and independent of the intensity of the incident light.
The rate of
the photoelectron emission can be measured from the current flowing through the
circuit or by the intensity of the of the light when frequency is constant.
Energy need
to extract an electron from a surface of a metal is called its work function,
the energy of photon is proportional to its frequency, according quantum
physics electromagnetic radiations consist of discrete particles called photon,
Photoelectric
effect is where classical physics fails because it fail to account for why photoelectrons are not emitted when
the frequency is below the threshold frequency. Why maximum kinetic energy is
less than the intensity of the incident light.
LAWS OF
PHOTOELECTRIC EMISSION
1. The number of electrons ejected is directly
proportional to the intensity of the incident light if the frequencies remain
constant. Therefore the velocity of of the escaping electrons is constant
whatever the intensity of light may be.
2. The velocity of ejected electrons increases with increase in
the frequency of the incident light but the number of electrons remain the same
if the intensity of the incident light remain the same.
No electrons are ejected when the frequency is below the threshold
frequency, ejected electron will remain at the surface of the surface if there are mo enough kinetic energy
to lift it up. The threshold frequency is the frequency enough to liberate an
electrons without giving them any additional energy.
No comments:
Post a Comment