2. CONCEPTS
PHOTO ELECTRIC EFFECT – discovered by Hertz
1887. It involves conversion of light energy into
electrical energy.
EINSTEIN PHOTO ELECTRIC EQUATION –
PARTICLE NATURE OF LIGHT. It is in accordance
with Law of conservation of energy . Max. KE =
photon energy – work energy
3. MATTER
WAVES- wave energy of particles.
The waves associated with the moving
particles are called matter waves also called
De Broglie waves .
Interference
, Diffraction , and Polarization
theories explain the wave nature of particles.
4. Action
PHOTO
CELL – It is technological
application of photoelectric effect . It
converts light energy into electrical energy.
DE
BROGLIE RELATION – De broglie
proposed the wavelength associated with a
particle of momentum P is given by
wavelenght=h/mv
5. DAVISON
AND GERMER EXPERIMENT –
Wave nature of electrons was experimentally
verified by Davison and Germer on 1928.
They observed diffraction effect with beam of
electrons scattered by a crystal. Both of them
shared Nobel Prize in 1937 for their
experimental discovery.
6. IMPORTANT POINTS
The
minimum energy needed by an electron
to come out from a metal surface is called
work function of the metal .Energy (greater
then the work function required for electron
emission from the metal surface can be
supplied by suitable heating or applying
strong electric field or irradiation it by light of
suitable frequency.
7.
Photoelectric effect is the phenomenon of electron
by metals when illuminated by light of suitable
frequency. Certain metals respond to ultraviolet light
while others are sensitive even to the visible light.
Photoelectric effect involves conversion of energy It
follows the law of conservation of energy. The
photoelectric emission is an instantaneous process
and possesses certain special features.
9.
The potential difference applied between the two
electrodesI3>I2>I1
photo
I2
I3
current
I1
Stopping pot.
retarding pot.
Collector plate pot.
10.
Effect of frequency of incident radiation on stopping potentialphoto
current
Vo’> Vo’’ > Vo’’’
stopping pot
(Vo)
Vo’
retarding pot.
Vo’’ Vo’’’
0
collector plate
11.
The stopping potential (Vo) depends on –
The frequency of incident light.
The nature of the emitter material. The stopping
potential is directly related to the max. kinetic energy
of emitted of electrons emitted :
eVo = 1/2mv²
max= kmax
Below a certain frequency , characteristic of the
metal, no photoelectric emission takes place .
12. Einstein’s
photoelectric equation is in
accordance with the energy conservation law
as applied to the photon absorption by an
electron in the metal. The max. kinetic
energy (½)mv²max is equal to the photon
energy (hv) minus the work function of the
target metal –
½mv²max =voe =hv =h(v-vo)
13.
The classical wave theory could not explain the main
features of photoelectric effect . Its picture of
continuous absorption of energy from radiation could
not explain the independence of KMAX on intensity,
the existence of Vo and the instantaneous nature of
the process. Einstein explained these features on
the basis of photon picture of light . Acc. To this ,
light is composed of discrete packets of energy and
momentum , which depend on the frequency of
incident light and not on its intensity . Photoelectric
emission from the metal surface occurs due to
absorption of a photon by an electron.
14. The
de Broglie wavelength associated with a
moving particle is related to its momentum p
as /p. The dualism of matter is inherent in
the de Broglie wavelength which contains a
wave concept and a particle concept. The de
Broglie wavelength is independent of the
charge and the nature of the material
particle.
15. Electron
diffraction experiments by Davisson
and Germer , and by G.P . Thomson , as well
as many experiments , have verified and
confirmed the wave nature of electrons. The
de Broglie hypothesis of matter waves
supports the Bohr’s concepts of stationary
orbits.
