PHOTOELECTRIC EFFECT
WHAT DOES PHOTOELECTRIC
EFFECT MEANS??
The
phenomenon of emission of
electrons from metal under the effect of
light is called photoelectric effect.
The electrons emitted by photoelectric
effect are called photoelectrons and the
current produced is called photoelectric
effect.
In this effect the light of short wavelength
is more effective.
HISTORY
 First observed by Hertz in 1887
 After one year , Hallwachs confirmed
by
an experiment by using zinc plates.
 In 1898 J.J. Thompson established
When light falls on a metal surface ,
electrons are emitted from the surface.
 Finally understood by Lenard in 1900
 Various experiments were conducted by
Lenard and Millikan.
THE PHENOMENON
DEPENDENCE ON VARIOUS
FACTORS
1. Light ray of definite intensity fall on plate P ,
photons are emitted.
2. plate C is positively charged, all these photons
reach plate C.
3. Increase in positive potential does not results in
increase in number of photons.
4. Increase in intensity of light results in increase
of photons.
5. Hence increase in the current.
1. If at stopping potential , light of frequency
higher than before be made to fall on P ,
current is established.
2. Higher the frequency higher will be the
maximum kinetic energy.
3. Lowest frequency which can emit photons
from a material is called threshold
frequency or cut-off frequency of that
material.
In 1905 Einstein explained photoelectric
theory .
The intensity of light depends on the
number of these photons.
When photons fall on a metal, it transfers
“whole” of its energy to any “one” of the
electron present in the metal and its own
existence is vanished.
Energy Of a Photon
1)
2)
3)
4)
5)
Rest mass = zero, but kinetic mass not
zero.
Photon travels with the speed of light.
Einstein's mass energy relation (E=mc
ˆ2)
Energy of photon E= hv/c^2
ADVANTAGES:
Used in turning on motor.
 Used for switching on & off street .
 Used in cameras .
 Used in industries.
 Used in solar cells for conversion of light
energy into electric energy.
