PHY SR DNR M03 S POTENTIAL.pptx

DUAL NATURE OF RADIATION
DUAL NATURE OF
RADIATION

Effect of  and I on stopping potential
Variation in
stopping potential
When  constant
And I varying
When  varying
And I constant

ip
0
V
VS3
VS2
VS1
Now, let us see the graph
of stopping potential vs
photoelectric current ip
Let intensity be constant,
let frequency
 be varying,   0,
As frequency increases
K.E. also increases
As kinetic energy
increases stopping potential
also increases.
But the saturation
current will be same for all
stopping potentials.
Because no. of photons
are same, no. of electrons
emitted will be same.
 the saturation current
will be same
We will see the effect of
frequency on stopping
potential
Saturation Current

ip
0
VS
Saturation current
V
I3
I2
I1
Let us see the graph between
stopping potential &
photoelectric current ip
We will see the effect
of intensity on stopping
potential
Let frequency be
constant, be varying i.e.
intensity no. of photons
be varying
If no. of photons
increases, the no. of
electrons emitted will
also increase.
If no. of electrons
increases, photoelectric
current will also
increase.
But KEmax will be
constant because
 is constant.
Therefore, for
different saturation
currents stopping potential
will be same.

1. For different saturation currents stopping potential will be ……
a) increasing
b)
c) same
d) changes
decreasing

2. If the number of photo electrons increases the photoelectric current…
a) increases
b)
c) remains constant
d) none of these
decreases

CONSTRUCTION OF PHOTOELECTRIC CELL
It consists of a
semi cylindrical photosensitive
metal plate E (emitter)
E
PHOTOELECTRIC CELL

C
It consists of a wire loop C
(collector)
E

Both emitter and collector are
supported in an evacuated glass
tube is quartz Bulb
E C

It is connected to the external
circuit having a battery and
a micro ammeter (mA)
E C
mA
+
–
Battery
+
–
Photo sensitive plate E
Collector plate C
Glass Tube

When light of suitable
wavelength (l) l < l 0 falls
on the emitter E
E C
mA
+
–
Battery +
–
WORKING
Collector plate C
Glass Tube
Photoelectrons are
emitted
These photoelectrons
are attracted towards
collector (C).
As the electrons get
collected by collector (C)
small photoelectric current ip is
induced.
ip
This photoelectric
current is recorded by
micro ammeter (mA)
If the intensity of incident
radiation is increased, the rate
of photoelectric current
emission increases.
As rate of photoelectric
emission increases current also
increases
Therefore, current is directly
proportional to the intensity
of incident light.

WORKING
Collector plate C
Glass Tube
E C
mA
+
–
Battery
+
–
ip
Now, connect any
electrical device to this
photocell.
Due to the photoelectric
current the device will be in
working condition.
Electrical Device
 The photoelectric cell
is a device which converts
light energy into electrical
energy

The rate of photo electric emission is directly
proportional to the intensity of incident
radiation.
APPLICATIONS OF PHOTO-CELL
Exposure Meter :
PRINCIPLE :
It is a device used along with a camera to find the correct time of
exposure for having a good photograph.
1)
2)

1) In this application, a photoelectric cell is used to activate
relay mechanism.
Burglar Alarm :
Photoelectric effect is instantaneous
PRINCIPLE :
2) The light from an infrared lamp falls on the photosensitive surface of
photoelectric cell .
3) The infrared beam is invisible and when some one crosses the beam, the light is
cut-off and photoelectric current stops.
4) This operates the relay circuit and starts ringing the bell.

2) The photoelectric current varies according to the intensity of light
transmitted by the amplified sound track.
3) The varying current is amplified and fed to the loudspeaker.
4) The loudspeaker reproduces the sound which is recorded on the sound track.
1) Light from a powerful lamp passes through the
sound track at the edge of the film and falls on
photosensitive surface of photoelectric cell .
Sound reproduction from motion pictures :

1. The photoelectric cell works on the principle of...
a) Raman effect
b)
c) both of these
d) none of these
photoelectric effect

2. In a photocell, increasing the intensity of incident light increases...
a) stopping potential
b)
c) max. kinetic energy
d) none of these
photoelectric current

3. The photocell is used to convert....
a) electric energy to light energy
b)
c) both of these
d) none of these
light energy to electric energy

Photo-electric emission occurs from the surface of a metal only
when the incident radiation has a frequency greater than or equal to
a certain minimum frequency (0) called the threshold frequency.
Characteristics of the Photoelectric Effect
The threshold frequency is different for different materials.
A)
B)
It follows that for photoelectric emission to occur, the wavelength
of incident radiation must be less than or equal to l0 (i.e.  greater
than or equal to 0).
C)
  0 l  l0

The wavelength (l0)
corresponding to the threshold
frequency (0) is called the
threshold wavelength.
l0 =
0
c

The number of electrons emitted per second from a given surface is
directly proportional to the intensity of incident radiation and   0.
The maximum kinetic energy of photo-electrons increases with
the increase in the frequency of incident radiation.
Photoelectric effect is an instantaneous process. There is no time lag.
D)
Characteristics of the Photoelectric Effect
E)
F)
Kinetic Energy does not depend upon the intensity of incident
radiation.
G)

Failure of Wave Theory to explain Photo – Electric effect
i) According to wave theory, the kinetic energy of the photoelectrons
depend on the intensity of the incident radiation, and increases with
increase in intensity.
ii) But according to photoelectric effect the K.E depends on
frequency and not on the intensity.
A)
B) The wave theory also fails to explain the instantaneous nature of
photo-emission.

1. Threshold frequency is different for ……..metals.
a) different
b)
c) independent
d) none of these
same

2. Kinetic Energy is independent of...
a) frequency
b)
c) wavelength
d) none of these
intensity

(2) e V0 = h–W0
The Einstein photoelectric equations are
(1) K.E.max = h–W0
mv2 = h –W0
i.e.
1
2
max
EXPLANATION OF CHARACTERISTICS OF PHOTO ELECTRIC
EFFECT WITH EINSTEIN’S PHOTO ELECTRIC EQUATION

Characteristics of photoelectric effect are :
For photo emission to take place, the incident light energy must
have frequency   0 if it  < 0 then photo emission will not take
place.
(1)
For a given photosensitive material and of incident light. ( 0 ) ,
ip  I.
(2)
EFFECT WITH EINSTEIN’S PHOTOELECTRIC EQUATION

For a given frequency ( 0 ), the maximum kinetic energy
linearly increases with increase in frequency , it is independent of
intensity.
(3)
The photoelectric effect is an instantaneous process and there is
no time lag.
(4)

According to Einstein's photoelectric equation
Energy of photon = Photoelectric + K.E.max
Work function
=
h W0 + K.E.max
 E = W0 = h0
at  = 0
 h = h0 + K.E.max
 h – K.E.max
h0 =
 h K.E.max
(–0 ) =
If  = 0
EXPLANATION OF CHARACTERISTICS OF PHOTOELECTRIC
EFFECT WITH EINSTEIN’S PHOTOELECTRIC EQUATION
 K.E.max = 0

When  < 0
K.E. becomes –ve
h ( – 0) K. E.max
According to Einstein's photo electric equation
And all know that K.E
cannot be negative
When   0 K.E. has some
positive value
=
When  < 0
no photo emission takes place
When   0
 photoemission takes place
with some positive value of kinetic energy

According to Einstein's equation we come to the conclusion that
1) When = 0 the photoelectrons are emitted without kinetic energy
2) When < 0 the photoelectrons are not emitted
3) When >0 the photoelectrons are emitted with some kinetic energy

h ( – 0) K.E.max
= As the I  the no. of photons
striking the metal plate 
As the no. of photons , the
photoelectric current ip 
This explains that ip  with
 in intensity
Here ‘I’ represents intensity
of incident radiation.

h ( – 0)
 The photoelectric effect is instantaneous
and there is no time lag
= Emission of photoelectrons is
a result of collision between
electrons and photons.
W0

1. If  < 0 , will photoelectrons be emitted...
a) yes
b)
c) may be
d) none of these
no

2. An increase in intensity of beam will _____ the photoelectric current.
a) increase
b)
c) no change
d) none of these
decrease

4. Maximum K.E of emitted photoelectrons depends upon...
a) frequency
b)
c) wavelength
d) none of these
intensity

Thank you…

PHY SR DNR M03 S POTENTIAL.pptx

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