Experimental Study of Photoelectric Effect Video Lecture - NEET

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what is photoelectric effect the
phenomenon of emission of electrons from
a metal by incident electromagnetic
radiation right so that is what we saw
till now photoelectric emission was also
the same thing emission of electrons by
incident electromagnetic radiation so
the electrons which are emitted in this
case are known as photo electrons and
the current constituted due to the flow
of these photo electrons is known as the
photo current this is how things happen
if this is your metal surface the metal
surface will obviously have the
electrons because every metal is has
free electron they are good conductors
right so when this electromagnetic waves
fall on this method the electrons get
ejected and this is photoelectric effect
substances emitting electrons were
illuminated by visible light are known
as photosensitive so those kind of
substances are known as photosensitive
that means if you give a light of any
suitable frequency somewhere around the
range of visible light they emit
electrons so they are known as
photosensitive materials
almost all letters they have I mean they
have pre electrons all metals are free
electrons and they are all sensitive to
some other other light and in depending
upon the electronic configuration of the
metals they will react to the light for
example they will react to their
corresponding frequencies for example if
we take the example of sodium sodium has
only one valence electron therefore that
one valence electron can be easily taken
out from the sodium so sodium means the
light of not very high frequency or
something like that so sodium is
photosensitive to visible light whereas
if we take the example of magnesium it
has got two free electrons so it needs
little more energy to take out those two
three
electrons out of it so magnesium is
sensitive to ultraviolet light which
which has a little more frequency than
that of the visible light so that is how
I mean all the metals are I mean they
show they can show photoelectric effect
but it will react to what kind of
frequency range that depends upon the
electronic configuration of the metal
because the electronic configuration of
the metal tells us about the work
function of the metal and that decides
the threshold frequency at which the
photoelectric effect will start taking
place let us now look at the detailed
experimental study of photoelectric
effect as I mentioned before that halvah
chant Lynott they wants us to know how
the photocurrent will vary with the
intensity with the frequency of the
incident light so what are the factors
that actually determine the photocurrent
so this experimental setup was used to
study this it looked somewhat like this
which has an evacuated tube as you can
see here this is the evacuated tube
inside this they had an ammeter this is
your amitabh the C denotes cathode and
the a denotes a node so the emitter is a
photosensitive plate when I talk about a
photosensitive change that means it is a
little more sensitive to visible light
it can easily emit electrons when light
for when it is illuminated by light
there is another metal plate which acts
as collector so we choose this emitter
in such a way that it is little more
photosensitive as compared to the other
metal plate right so now we have we use
a monochromatic light of short
wavelength okay let me have a battery
which maintains the to maintain to
create an electric field inside the tube
or and to maintain a potential
difference between the two plates we
have a voltmeter to measure the
potential difference between
the two plates and we also have an
ammeter which will measure the amount of
current flowing through the circuit that
is which will measure the photocurrent
touch so in this case what happens when
this monochromatic light falls on this
amitabh electrons will get immediate
right so what happens when the light
falls on this photosensitive plate
electrons get emitted from this they get
attracted towards the other metal plate
that is a collective and due to the flow
of this electron from the photosensitive
plate towards the metal plate electric
current gets constitutive and this
electric current is nothing but the
photo current and the photo current is
measured by the amateur so it was a very
simple experiment which was carried out
but this experiment was helpful in
studying the variation of photo current
whereas we change the frequency of the
incident light as we change the
intensity of the incident light as we
change the potential difference between
the two plates so we will see the
conclusion of this experiment one by one
so the first conclusion which we will
talk about is the variation of photo
current with the intensity of radiation
what happened when we change that when
we tried changing the intensity of the
incident light how did the emitter
response I mean what was the magnitude
of the current flowing through the
circuit it was it was done like this the
frequency of the incident light was kept
fixed the potential difference between
the two electrodes but was kept fixed so
the only thing that was varied in this
case was the intensity of the radiation
so when we increase or decrease the
intensity of the radiation it was
observed that the photo current
increased linearly with the increase in
intensity as we increase the intensity
the photo current also increases with
time now what does this photo current
actually photo current is nothing but
the number of photo electrons per unit
time right what is current current is
nothing but number of electrons flowing
per second similarly it is now
photoelectrons per second right so that
is photocurrent
so it was saying that as the intensity
of the incident light was increased the
number of photoelectrons emitted also
increased with time so if the number of
photoelectrons emitted in keep
increasing the photocurrent will also
increase right
so photocurrent varies linearly with the
intensity of light so we want more
photocurrent we have to provide light of
more intensity the next one was
variation of photo current with
potential that means what happens if we
change the potential difference between
the two plates will the photo current
remain the same or it will change and if
it will change how will it state so well
then we talk about this potential way to
talk about two types of potential one is
a positive potential and the other one
is a negative potential one is positive
potential and the other one is negative
potential what is positive potential
positive potential is the scenario when
the collector this is my collector and
this is my emitter so this is my
photosensitive plate where light is
falling and this is the collector where
the electrons are coming now if this is
positive with respect to emitter that is
positive potential when we see that we
are talking about positive potential how
photo current is varying with positive
potential that means we are keeping the
collector at a higher potential as
compared to the emitter when we talk
about negative potential I mean that the
collector is at a negative potential
with respect to the emitter so we are
keeping the collector at a lower
potential as compared to the emitter so
this I just explained you the terminal
Rajas because now when we discuss about
the variation of photo current we even
use these two terminal Rajas right now
this was how it looked like if we plot a
graph this is my origin
now as the positive potential keeps
increasing I mean first you consider the
first scenario first you consider the
scenario of positive potential positive
potential that means the collector is at
a positive the focus that this collector
is at positive with respect to that
emitter so the metal plate is positive
with respect to the photosensitive plate
so since it is highly positive so
whatever electrons are coming out from
this eMeter day will get attracted
towards it so the electrons will keep
flowing towards the collector that means
as the positive potential is increased
more and more electrons will keep
flowing towards collector so the photo
current will increase so as you can see
initially the photo current is
increasing but gradually if you see the
current saturates so this is a
saturation level that means beyond this
the current is not increasing even
though the potential keeps increasing
but the current remains the same why
does this happen
this happens because this is also a
mater right so this metal also has some
tricks number of electrons so a time
comes when all the free electrons of the
photosensitive plane has already moved
towards the collector like in that case
even if you keep increasing the positive
potential there is no point right there
are no more photo electrons to move
toward to flow therefore the current
saturates it cannot increase beyond that
whatever photo electrons were there that
is moving beyond that it is not moving
now why is it happening like that we
will talk about all these things when we
will try to explain this photoelectric
effect right now we are only talking
about the experiment which was performed
to see the variation of photo current by
changing different parameters so we will
just look at the results of experimental
so the result was that as we keep
increasing the positive potential photo
current increases with increase and
positive potential till the current
reaches a saturation value so this was
the first observation now what was done
in the second part of the experiment
what was done the the collector
was kept at a negative potential with
respect to the emitter right so now the
collector is at a lower potential and
now this negative potential is also
sometimes known as retarding potential
now when this negative potential is
increased that means the potential value
is increasing but on the negative side
now when this was increased it was found
that the current kept decreasing this is
because now this is negative now if this
is negative the electrons which is
coming out from the emitter
they will get repelled by the collector
so many of the electrons will get
repelled and they will go back to the
emitter but there will be some electrons
who will still be able to reach the
collector hole will be those electrons
the one who has more energy that means
earlier all the electrons which were
emitted by the emitter was getting moved
towards the collector right now what
will happen many of the electrons will
get repelled by the collector because
the collector is at a lower potential
but some of the electrons which have
maximum energy which have maximum
velocity they will still be able to
reach the connector so that means the
current will not drop to zero but the
current will decrease because earlier
the contribution towards current was by
all the photoelectrons but now the
contribution has decreased because some
of the electrons are getting repelled by
the collector so they are not moving
towards the collector so now lesser
number of photo electrons are moving
towards the collector therefore the
photo current is lesser and it was found
that atom at a particular value of
potential the photo current became 0 so
this particular value of the potential
or the name regarding potential is known
as the stopping potential so the
stopping potential is nothing but the
potential at which the photo current
drops to zero that means beyond this
potential there is no flow of current at
all that means what happens at this
talking potential at this stopping
potential even the most energetic
electron that means the
Tron which add the maximum kinetic
energy even that electron is failed to
reach the collector so when none of the
electrons because earlier the situation
was let us suppose initially when this
collector was at a positive potential at
that time if hundred electrons were
reaching the collector when they kept
the collector at a negative potential
not hundred some 70 were able to reach
let us suppose 30 we're getting repelled
now as I keep increasing the negative
potential the number of electrons
reaching the collector will gradually
decrease from 70 to 60 to 50 to 40 at
last it will become one that last
electron which is able to reach the
collector that is the electron which had
the maximum kinetic energy but if you
still keep increasing the negative
potential that an electron with the
maximum kinetic energy will also not be
able to reach the collector so there we
get the stopping potential that means
there is no electron which is actually
reaching the collector and as I desire
there is no current flow therefore the
photo current is zero right so when we
talk about this stopping potential what
happens when the photocurrent rocks to
zero actually the maximum kinetic energy
of the photoelectron becomes equal to e
into V naught when V naught is the
stopping potential so the charge on the
electron multiplied by the potential
applied becomes equal to the maximum
kinetic energy of the most energetic
photo electron so now even the most
energetic electron cannot reach the
collector because as long as there were
some current flowing this kinetic energy
the maximum kinetic energy was greater
than this right so I hope you understood
how we arrived at this expression and
what is the concept of stopping
potential okay so the second part was
photo current decreases with increase in
retarding potential till it becomes zero
so these were the two important
observations from this photoelectric
effect experiment so now let us quickly
talk about stopping potential we will
just review and I already
tell you the concept stopping potential
is the minimum negative potential or
retarding potential given to a plate for
which the photocurrent becomes 0 it is
also known as cutoff potential because
at this point the photocurrent is
getting cut off lettuce is cut cut off
potential for a given frequency of
incident radiation stopping potential is
independent of the intensity let us
suppose if the frequency is kept
constant and if you keep changing the
intensity of incident light the stocking
potential will remain the same
so that means stopping potential depends
on frequency of the incident radiation
and it does not depend upon intensity so
stopping potential depends on frequency
and it does not depend upon intensity we
will look at it also graphically so what
are the factors on which it depends it
depends on frequency of the incident
radiation it also depends on the
material of the emitter plate see
material of the emitter plate will
always play a very important role
because it is the material of the plate
which decides how much is the work
function how much energy is actually
required to be given to emit an electron
therefore the threshold frequency also
depends on the material of the plate the
work function also depends upon the
material of the plate and now the
stopping potential also depends upon the
material of the plate because end of the
day everything depends upon the
electronic configuration of the metal
which is being used how eager or how
difficult it is for the metal to lose
its valence electrons so everything
depends upon that right so I hope I am
able to explain you what is the concept
of stopping potential now let us quickly
look at what we spoke just not how is it
that stopping potential does not depend
upon intensity and it depends upon
frequency so a plot before I told you
how photocurrent values with potential
in this case you see this is my positive
pretend
she'll decide this side my negative
potential is increasing so if you see
four different intensities of light we
used three different intensities of
light I 1 I 2 and I 3 and we saw when we
plotted their graph and we actually
perform this experiment sight and we saw
them we measured the values of
photocurrent by increasing the positive
potential again by increasing the
negative potential so when we plotted
them on a graph we saw that the value of
stopping potential for all the three
intensities was the same so this shows
that stopping potential does not depend
upon the intensity of light right so
this graph shows that stopping potential
does not depend on stopping potential
does not depend on intensity of the
incident light right but what depends
upon the intensity of incident light the
saturation current if you look at it for
I 1 this is the saturation current for I
2 this is the saturation current and for
I 3 this is the saturation current so
from here we see that the saturation
current increases they increase in
intensity that's because we chose before
Auto right when the intensity of the
incidentally increases our conclusion
one was that the photo current increases
with increasing intensity so in all the
three sets we have increased the
intensity therefore the photo current
has increased and photo current is
nothing but the saturation current is
nothing but this photo current rate the
maximum value of this photo current
therefore each time we increase the
intensity the value of maximum photo
current also increases so saturation
current depends upon the intensity of
incident light but stopping potential
does not depend upon the intensity of
incident light so so far we discussed
about two things dependence of photo
current on intensity
and dependence of photocurrent on
attention now the curve conclusion was
the variation of photocurrent with
frequency of incident radiation we will
now see if the photo cannon changes when
we change the frequency of the incident
radiation now for this also experiments
were performed for three different
values of intensity like for three
different values of frequency like how
we performed it for intensity similar
experiment work performed by taking
different values of frequency so first
we took then incident light of frequency
nu 1 then new to the Nutri and then
similarly we first increased the
positive potential and measured the
values of photoelectric current then we
increase the negative potential and
measure the values of photoelectric 1
current at each point and then we
plotted the graph and what did we see we
see that the saturation current is the
same for all different frequencies that
means saturation current does not depend
upon frequency but the values of
stopping potential changes with
frequency as the frequency in this case
if you see nu 3 is greater than mu 2 is
greater than mu 1 that means for maximum
frequency this is this is the maximum
frequency for maximum frequency the
stopping potential is greater and for
minimum frequency the stopping potential
is listen that means stopping potential
that is V naught is directly
proportional to the frequency of
incident light right so we saw two
things saturation current depends upon
intensity and stopping potential depends
upon frequency so this was one
observation from this now what about
photo current what is the variation of
photo current to be frequency the photo
current if you look at clip here photo
current does not depend on sequence e
match the because the saturation current
also if you see it is not
I mean it is the same for all different
frequencies the frequency doesn't play
an important role in changing or in
increasing the photocurrent but it
changes the stopping potential so
frequency as you keep changing the
frequency if you want more current you
have to get change the intensity but if
you want to change the stopping
potential that means if you want to
change the change the duration for which
the amount of photo current should flow
in that case you have to change the
frequency because stopping potential
decides for how long
photo current will be there for how long
electric current will flow right so when
we plot the graph of this stopping
potential with frequency we see that
eating reasons with increase in
frequency but initially it is zero for
some time that means this value of
frequency is the threshold frequency
below below which no photoelectric
emission took place so the experiments
told that there was a minimum value of
frequency above which the photoelectric
emission took place below that value of
frequency photoelectric emission did not
happen at all and when we talk about the
amount of photo current amount the photo
current can be governed only by
intensity it is not dependent on
frequency if you think that that by
increasing the frequency you can
increase the photo current that is not
possible because increasing the
frequency will affect the stopping
potential it can make the amount of
current whichever is flowing to flow for
a longer time it will drop to zero after
some time because it will increase the
value of the stopping potential so the
time when the photo current rocks to
zero that will come little later because
that will need a little more potential
like so this is how we studied the
variation of photo current with
frequency so this introduced again a new
term that is Thresh
treatments it is the minimum frequency
of incident radiation below which
photoelectric emission does not affect
so it is also known as cutoff frequency
so the way we spoke about cutoff
attention where the photoelectric
emission does not affect so that was the
potential corresponding to which there
was no photoelectric emission this is
the frequency corresponding to which
there is no photoelectric emission and
you saw that both are related like
because that potential the cutoff
potential is actually determined by the
frequency it varies with metals I don't
think I need to explain this point again
because every methyl will have different
electronic configuration so let us have
a quick conclusion of the experimental
phase performed on photoelectric effect
so taking into conclusion one two and
three together what all did we see we
saw that photo current is directly
proportional to the intensity of
incident light if we increase intensity
current will increase saturation current
is proportional to the intensity of
incident light so all currents are
proportional to the intensity right
stopping potential is independent of
intensity stopping potential has nothing
to do with intensity there exists a
threshold frequency for every material
below which no photoelectric effect
takes place stopping potential and
maximum kinetic energy of photoelectrons
increases linearly with frequency so
this I have told you very clearly how
stopping potential increases with
frequency however what about this
maximum kinetic energy I mentioned
before right what a stopping potential
that means a potential where there is
the photo current will drop to zero that
means at stopping potential the electron
which has the maximum kinetic energy
that electron will also not be able to
reach the collector electrode right that
means at stocking potential the energy
due to the work done by the stopping
potential becomes equal to the maximum
kinetic energy of the photoelectron okay
that's what I had mentioned before that
is K Max becomes equal to e v-not so now
if V naught that the stopping potential
if it increases linearly with frequency
so this maximum kinetic energy will also
increase with frequency what does this
maximum kinetic energy it is basically
the maximum kinetic energy that a
photoelectron can have like when light
falls on the on the emitter photo
electrons are emitted so what is the
maximum kinetic energy that the photo
electron can have it is that maximum
kinetic energy photoelectric effect is
an instantaneous process the experiment
also shown showed that as soon as light
fell on the emitter plate electrons got
emitted so there was no time lag in
between as soon as light fell
immediately electrons but immediate
so it was an instantaneous process so
this was photoelectric effect
experimental II I mean experimental II
all these when the observations now
whatever we see experimentally they are
always true correct because that is
something which is happening it is not
an hypothesis it is not an imagination
it is something which is happening in
reality that means whatever is written
on this screen right now they were all
experimental II observed and moved
experimentally it was calculated and
proved that okay saturation current is
not depending on intensity for different
intensities they tried it similarly for
different frequencies they did the
entire experiment and then they found
that stopping potential is dependent on
frequency so all these things are true
right so now the question is
photoelectric effect is M is a
phenomenon of light light is falling on
something and it is causing electron
emission now till now the scientists
have told that light is of wave nature
so if light
of wave nature then wave nature of light
should be able to explain why this photo
electric effect so the wave nature of
light should be able to explain properly
all these six points which are listed on
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