really we'll see it's 1887 and Hinrich Herz is about
to make history he turns out his primary
coil producing a very strong spark over
here and almost immediately a second
spark appears in his handheld coil
electricity had jumped through the air
he had demonstrated for the very first
time the existence of electromagnetic
waves one of the core predictions of
Maxwell's equations but little did he
know that soon he would stumble upon
something totally by accident that would
shake the very foundations of
physics see while playing with these
Sparks he noticed something almost by
accident that shining ultraviolet light
on the metal made the spark length
bigger he had no clue what was going on
but he probably didn't care much because
he was celebrating the discovery of
electromagnetic waves and was probably
too busy having a unit named after him
but soon the electrons were discovered
and it then became clear what was going
on the ultraviolet light ejected
electrons from the metal this made the
spark bigger and it was called the
photoelectric effect and this led
Einstein to revolutionize our
understanding of light eventually
earning him the Nobel Prize right right
well when I first learned about the
photo Elric effect I was wondering what
the big deal was why did Einstein win a
Nobel Prize for explaining it I mean I
can explain it see light has energy and
that energy is absorbed by the electrons
and the electrons use that energy to
escape from the nucleus giving you
photoelectric effect there you have it
where's my Noel price and more
importantly it said that this effect LED
Einstein to develop his theory of
photons but how exactly and was it
really Einstein who developed the idea
of photons wasn't it this guy Max car
erns Ludwick plunk who came up with this
idea while studying the black body
radiation I mean e is equal to H new or
h F literally has planks constant in it
so why do we credit Einstein for the
discovery of photons none of this made
any sense that is until I dug into the
details of all the history and all the
experiments that went behind it then my
mind was truly blown so if I do this
right then by the end of this video we
would have rediscovered step by step all
the thought processes and scientific
models that slowly and steadily evolved
which led to the quantum revolution of
light so if you're ready for this let's
begin
einen says mes you are absolutely right
initially even we thought what's the big
deal with the photoelectric effect right
that is until a man named Philip Leonard
who was Hinrich her's assistant made a
shocking Discovery what was the
discovery Einstein Einstein asks mes if
you were to increase the intensity of
light over here what do you think will
happen to the speed at which the
electrons are coming out and I'm like
well if you were to increase the light's
intensity then you're pumping in more
energy electrons are going to absorb
more energy so they will come out with
more kinetic energy they'll come out
faster obviously and that's exactly what
Leonard thought but Leonard wanted to
win a Nobel Prize he wanted to make his
Mark in the scientific community so he
says hey why don't I experimentally test
this and I'm like wait a second how are
you going to experimentally measure the
speed at which electrons are coming out
I mean it's the
1900s well Leonard comes up with a
brilliant idea if a baseball is moving
very slowly with a little bit of kinetic
energy it takes a little bit of effort a
little bit of energy to stop it but if
that same baseball was moving much
faster with more kinetic energy it takes
a lot more energy to stop it so by
figuring out how much energy it takes to
stop something you can estimate its
kinetic energy okay so if you could
figure out how much energy it takes to
stop these electrons then I can estimate
the kinetic energy of these electrons
that makes sense but how in the world
are we going to try and stop the
electrons how would we know how much
energy took to stop the electrons
Leonard's brilliant idea is to use a
negative voltage think about it
electrons are coming out from here right
now if you use another metal to collect
the electrons which we're going to call
the collector but put a negative voltage
to The Collector then that will repel
the electrons and it can make the
electron stop now if it takes a little
bit of voltage to make the electron stop
that means the kinetic energy of the
electrons was very little but if it
takes a lot of voltage to make the
electron stop that means the can
electrons had much bigger kinetic energy
makes sense right and I'm like yeah that
makes sense but how do I know if the
electrons have stopped or not like I
can't see them right ah this is where
you put an amp meter if the electrons
are collected then there will be a
current and the Amit will detect it in
fact in this particular example there is
a current that means I know the
electrons have not stopped so this much
voltage is not enough so let's turn up
the voltage until the current just stops
so at this point I can now visualize
that all the electrons that are coming
out over here must have stopped and must
have turned around none of them are
getting collected so in this case it
takes three volts to stop these
electrons right now imagine another
photo experiment in which it took say
six volts to stop the electrons then I
can immediately compare and say that hey
hey hey there the electrons must have
come out with more kinetic energy in
fact about twice as much kinetic energy
as over here because it took twice as
much voltage to stop it this means this
stopping voltage we as we call it is a
direct indicator of the kinetic energies
with which the electrons are coming out
and looking at this I'm like Bravo
Leonard Bravo that's truly brilliant so
what was the shocking Discovery what did
you find all right now mahes what do you
think should happen to the stopping
voltage as you increase the intensity of
light and I'm like as we increase the
intensity of light since electrons will
come out with more kinetic energy it
should be harder to stop them ah the
stopping voltage should increase exactly
but when Leonard did the experiment what
he found is that increasing the
intensity of light had no effect on the
stopping voltage the stopping voltage
did not change at all regardless of how
bright he made the light I mean of
course we're dealing with UV light you
can't really see it he was using a lot
of filters to change its brightness and
change its intensity but you get the
point increasing the intensity has no
effect on the dropping voltage and the
big question was why how does it make
any sense I'm pumping in more energy and
yet electrons are not coming out with
more energy how does that make any sense
now of course Leonard did find that
increasing the intensity increased the
number of electrons that were ejected
and that kind of probably makes sense
right like you shine more light and
you'll get more electrons so that's fine
but the big question was why they coming
out with more energies the stopping
volage did not change at all so why is
it that if I input more energy for some
reason electrons don't come out with
more energy I get more electrons but
they pretty much come up with the same
energy as before why is the intensity
has no effect on the kinetic energies of
the electrons was a big big question
mark So Einstein this is where you come
in right right and Einstein says No in
fact Leonard actually came up with a
brilliant explanation for this as well
Leonard says mahes think about a cannon
if you light it on fire does the size of
the fire whether small or big decide how
fast the Cannonball flies out I'm like
of course not it depends upon the
gunpowder and stuff over here Leonard
says exactly the fire just triggers the
explosion it doesn't transfer energy
into the Cannonball and that's why the
speed of the Cannonball has nothing to
do with this fire it depends upon the
gunpowder and all the properties of the
cannon in a similar way maybe light is
only triggering the electrons maybe
light doesn't really transfer the
energies into the electrons maybe the
electrons already had these energies to
begin with while being inside the metal
maybe in some other form maybe light
only triggers it that would explain why
light's intensity has no effect on the
kinetic energy of the electrons the
kinetic energy of the electrons must be
an intrinsic property of the metal
itself not the light having more light
now it's kind of like having more fire
you can trigger more cannon balls
similarly if you have more light over
here you'll trigger more electrons and
that's why more electrons come out but
again their energies would have nothing
to do with the light itself looking at
this I'm like bra Leonard Bravo again
and I absolutely love it because we get
to see how science progresses you make
come up with ingenious experiments and
then come up with hypothesis to explain
it this by the way was called the
trigger hypothesis and I have absolutely
no idea that it existed my books didn't
cover this but anyways Leonard wanted to
be absolutely sure about his hypothesis
so he ConEd even more exper expent and
soon he found his trigger hypothesis to
be in trouble this time leard
experimented with different colors or
different frequencies of light higher
frequencies like X-rays and even lower
frequencies like visible light now
according to the trigger hypothesis
light is only triggering the electrons
so the energies with which electrons
come out should only depend on the metal
right so this means the frequency should
have no effect on the kinetic energies
of the electrons right right but what
leard found was that was not true
Leonard found that the high frequency
light actually made the electrons come
out with more kinetic energy and lower
frequency light made them come out with
lesser kinetic energy and if light's
frequency was too low then electrons
didn't come out at all no matter how
bright you made the light this means the
electron's energy not only depends on
the metal but it also depends on the
light not its intensity but its
frequency higher the frequency more
kinetic energy so it's doing more than
just triggering the electrons the
trigger hypothesis was in trouble
Leonard was in trouble he tried his best
but now we really needed to bring out
the big stuff we needed a revolution
kind of like the revolution that we had
okay happy website building now back to the video this is where Einstein comes
in so Einstein what went through your
mind when you went through the results
of the photoelectric effect and I says
nothing nothing initially but luckily a
few years ago he had gone through
another paper by Max plon on the black
body radiation connecting the dots from
that experiment and this one inspired
him to come up with something truly
revolutionary but what is it what did he
find well the details of the black body
radiation and how plunk solved it is a
story for another video it's also very
interesting so let me know but for the
purpose of this video to understand how
it inspired Einstein we don't need to
get into the details we can use a
brilliant analogy to get the essence of
what happened okay light's energy is
like this water smooth and continuous
you can transfer any amount from one cup
to another like you can transfer one m l
or 23675 ml whatever you want right but
when we treated light like this where
you can transfer any amount of energy
from one object to another our theories
couldn't explain the experimental
results again the details of that
experiment is not really important but
what's important is that try as you may
if you allow water to continuously be
transferred from one cup to another you
cannot explain the experimental results
so when finally plank stumbled upon this
he too struggled to fit the experiments
just like all the other physicists did
and so in later what he called an act of
Despair he came up with a wild idea
instead of a continuous transfer what if
you could only transfer water in
specific amounts like a minimum of one
spoon so you can transfer one spoon or
two spoons Worth or any multiples of
spoons worth of water but not say three
and a half spoons of water for example
and immediately I'm like plank why would
you do that well plank says it's a
counting trick okay if you allow the
water to transfer continuously I cannot
count it but if I do it one spoon at a
time now I can count how many spoons of
water has transferred and in fact and
you can literally see it in this paper
that he's using it as a counting trick
my very second plankk how big should
that spoon be ah plank says this is very
realized if the spoon size dependent on
the color of the water so if you have
high frequency water you have bigger
spoon size and low frequency water
meaning smaller spoon size then he could
probably explain the experimental
results so he comes up with this random
condition that the minimum quantity of
water that you can transfer the quanta
the size of the quanta should be
proportional to the frequency of the
water frequency of light and again
remember he's only doing this to fit the
experimental results it has no physical
reason whatsoever and I'm like plunk
what's the point even if you're
successful in explaining the experiments
how will you explain why this is true
why you have to transfer some minimum
amount and that minimum amount that
minimum quantity depends on the
frequency of water well you have to give
some explanation right and plank says he
doesn't have to this is where his final
step his master stroke comes in plank
says if this all works out if he can
explain the experimental results then he
will just set his proportionality
constant to be zero which means in
reality there is no minimum quanta there
are no spoons it was all a counting
trick after all water is just continuous
and he doesn't have to explain
anything brao plank Bravo did you like
the analogy I mean I loved it because it
captured the essence of what PL was
trying to do without having to go get
into the details of the black body
Edition and all of that but now the big
question is did it work and pl plank
says it half worked his strategy worked
perfectly and he was able to perfectly
explain the experimental results yay but
he wasn't able to get rid of his
constant he found out to make all of
this work that constant had to have a
particular value and that value is this
yes very close to the Plank's constant
accepted value that we have today but
think about what this means this meant
that it was no longer just a counting
trick it was physical reality that
nature is indeed transferring water in
some minimum Quantas but plunk hated
this idea that was never his intention
he never wanted to quantize anything if
water is continuous why is nature
forcing you to you know restricting you
to transfer it one spoon at a time in a
multiples of a particular spoon size why
was nature doing that it just didn't
make any sense to him this is where
Einstein jumps in and let's look at how
he connects the dots now Plank's energy
Quantas depend on the frequency and now
even in photo electric effect we again
find electron's kinetic energy also
depends on frequency can this be a
coincidence he thought no so he came up
with a much cooler idea what if light is
not like water at all but instead like
ice cubes then we don't have any choice
but to transfer one Cube or two cubes or
in multiples of one single Ice
Cube so here's the difference between
how Plank and Einstein thought according
to plank light is like water smooth and
continuous but for some reason nature
only transfered in discrete Quantas it
didn't make any sense to him why would
nature do that he didn't believe in
Quantum anything in fact he was so anti-
Quantum he didn't even believe in atoms
but it was Einstein who took it to a
whole new level Einstein says buddy I
can give you a reason for why nature
might be doing that what if light is not
like water smooth and continuous what if
light itself is made of these discret
Quantas like ice cubes then she has no
choice but to transfer one Cube at a
time so look Einstein gave a physical
interpretation of what plunk had
discovered when he was trying to do his
math counting trick and these discrete
chunks of light is what Einstein called
the photons so you can clearly see it
was Einstein who came up with this
revolutionary idea of photons and not
plunk Bravo Einstein Bravo truly out of
the box thinking but the big question
now is can this explain the
photoelectric effect better than the
trigger hypothesis let's find out
according to Einstein light is not made
of waves instead of particles like ice
cubes and he says when you shine light
on Metals the electrons have to absorb
one entire quanta the entire Photon you
cannot absorb half of it okay so what
happens now if electron absorbs the
entire Photon now if the photon doesn't
have enough energy to kick that electron
out well then the electron will just
Reit that Photon back and now even if it
absorbs another Photon like this again
it will Reit it back so look if your
Shining Light whose Photon energies are
too small to kick that electron out you
will not get photo electric effect and
if you were to increase the intensity of
light what you're really doing is just
increasing the number of photons but
since a single Photon doesn't have
enough energy to kick it out regardless
of how many photons you shine on it
nothing's going to happen it's kind of
like trying to knock over this golf ball
using mosquitoes it doesn't matter how
many mosquitoes you throw at it that
golf ball is not going anywhere but of
course one question we could ask
Einstein is why can't an electron absorb
multiple photons at the same time and
then enough energy to escape it and is
says I don't know he didn't have an
answer for that but today we do see
photons and electrons are quantum
objects incredibly tiny particles so the
chances of an electron absorbing two
photons at the same time that is
incredibly incredibly small so it can
happen if you have insanely High number
of photons you know incident per second
and that can only happen at very large
uh intensities of light and we have seen
multiphoton absorptions in such large
intensity lights but as long as the
intensity is not so freakishly High then
the probability of that happening is
just too small so this means for
photoelectric effect the photons should
have a single Photon should have a
minimum amount of energy to kick the
electron out and since the energy of a
photon depends on its frequency that's
the reason why you know you need a
minimum frequency for photic effect to
happen if the frequency is too low the
photo energies become too low and the
electrons won't get kicked out and now
as you increase the frequency the energy
of the photon becomes high enough that
it can kick out an electron and now if
you further increase the frequency then
the energy of a single Photon becomes
even more so when the electron absorbs
it gets even more energy and that's why
it will come out with even more kinetic
energy this now perfectly explains why
as the frequency increases since the
photon's energy increases it transfers
more energy to the electron and that's
why the kinetic energy will increase
that's why the kinetic energy or the
stopping voltage depends on the
frequency of light and of course if you
were to increase the intensity what does
it do well it just increases the number
of photons so that won't affect the
energy transfer to a single electron but
now since there are more photons more
electrons will be emitted and that
explains why increasing the intensity
gives you more electrons the photon
Theory beautifully explains the
photoelectric effect Einstein Take a Bow
your theory must have been now accepted
in the scientific Community super easily
with barely any inconvenience right and
I says actually no mahes it was really
hard in fact even for more than a decade
people didn't believe in the photon
Theory and I'm like what how does that
make any sense this Theory explains the
photo Elric effect way better than
trigger hypothesis ever did what's going
on there were many hurdles first of all
it challenged the already established
wave model of light we have Century
world of experiments confirming that
light is a wave like interference and
defraction and so much more and now this
Paton clerk comes out of nowhere and
says that hey all of that is wrong and
light is not a wave it's made of
particles and you expect people to
accept that no ways in fact this idea
was so so so revolutionary and people
hated it so much that people actually
thought that this was Einstein's biggest
blunder I mean take a look at this
recommendation letter written by Marx
plon in 1913 you can pause and read this
so he Praises Einstein over here and
then he says that he may sometimes have
missed the target of his speculations as
for example in his hypothesis of light
quanta cannot really be held against him
oh man this age well but secondly the
photon Theory challenged a noble laate
who are we talking about
Leonard that's right Leonard actually
won a Nobel Prize in 1905 the same year
Einstein published all his beautiful
papers on special relativity and and
photo Elric effect and uh Brownian
motion and all of that and Leonard won
it not for photo Elric effect he won it
for his contributions to you know
discovery of electrons he did a lot of
work over there but anyways with the
Nobel Prize came a huge ego so a mere
Pon clerk had dared to challenge a noble
laurate that triggered Leonard so he
became a full-blown anti- Quantum anti-
relativist anti- Jews and anti Einstein
and he had a huge influence on the
scientific Comm
so Einstein this is really bad who came
to your rescue then now enters our final
character Robert milikin a huge
experimental physicist and I'm like
that's the guy who discovered the charge
of an electron using the oil drop
experiment was he in the favor of your
theory and Einstein says no he was in
fact one of the biggest critics and yet
he was he played a crucial role in
validating the photon Theory completely
unintentionally milikan was was not only
against the photon Theory like most
other people but he found something
extremely weird in his 1905 photo LC
paper so weird that he said Einstein not
only am I against you but I'm going to
experimentally prove it to you that you
and your foot on theory is absolutely
wrong he was absolutely convinced that
it is wrong but what did milikan find so
weird in his paper I mean the photon
Theory made perfect sense to me it
nicely explained the photolytic effect
so what was so weird and I says M yeah
there was something weird in my photo
paper there was a bold claim that it
made and even I was a little nervous
about it what what was it and says all
right mahes for this we need to do a
little bit of math and I'm like no not
the math and says mahes don't worry
about it the math is going to be super
intuitive if you want take a break and
then come back because this is it this
is the climax This Is The Moment of
Truth so if you're ready let's do it all
right welcome back we have a ball and
we're going to kick it up giving it 100
units of energy our goal is to figure
out how much kinetic energy the ball
would have when it comes over here
that's it that's all we need to figure
out okay now as the ball goes up it's
going to lose some of that energy to
gravity because it's doing work against
gravity isn't it so let's say that work
that it does the energy that it loses
Against Gravity is 30 units so how much
energy is left over here well 100 - 30
70 units that's the kinetic energy isn't
it and that's it that's all there is to
it so we can now write down an equation
the kinetic energy of the ball when it
reaches over here is going to be the
energy which which you kicked it minus
the energy that got used up to overcome
gravity makes sense right I'm like yeah
that makes a lot of sense now my sense
say okay what if you had a lot of such
balls at different heights then which of
the ball mahes would have the maximum
kinetic energy when it reaches over here
and I'm like well it's got to be this
one because it's the closest so it has
to do the least amount of work against
gravity and I say like exactly so the
ball that has to do the least or the
minimum amount of work against gravity
will end up getting the maximum amount
of kinetic energy Ein says mes this is
my photo Elric equation of course I
wrote it using different symbols back
then but this is it this is the photo
Elric equation because now if you
replace your basketballs with electrons
you get the same idea if you supply
energy of a photon to them that is e
over here the electrons will use up some
of that energy to overcome not gravity
but electric forces and then whatever is
the remaining energy that would be the
kinetic energy with which they come out
and the electron that is close to the
surface is the one that needs to do
minimum amount of work to overcome the
electric forces and therefore it's those
electrons will get the maximum kinetic
energy okay that looks pretty
straightforward to me what's so weird
over here and is says mes let's simplify
this a little bit and then you'll get it
suppose the maximum kinetic energy
happen to be one electron volt mahes do
you know what it means and I'm like I
remember studying about electron volts
but I forgot it einon says don't worry
it's a unit of it's a unit of energy and
one electron volt basically means you
need one volt to stop that electron
that's all it means okay so if you apply
one volt that electron is going to stop
in other words the stopping voltage is 1
volt if that electron if that maximum
energy of the electron was three
electron volts then that means you need
three volts to stop that electron in
other words look if you know the
stopping voltage which you can calculate
experimentally then if you multiply it
by E you will now find the maximum
kinetic energy of the electrons that are
coming out I find that fascinating
because you can now do big macroscopic
experiments to figure out to measure
microscopic quantities that's how it all
works which is incredible anyways so
this means I now know how to find the
maximum kinetic energy just multiply
stopping voltage with e now that equals
we know this is the energy of the photon
so energy of the photon is H into F
minus this minimum work needed minimum
energy needed to overcome the electric
forces and Einstein ask now do you see
it and I'm like no what is so weird over
here I can't find out and Ein says all
right don't worry let's divide the whole
equation by e that's the final step so
if we do that if we divide this whole
equation by E we will now get V stop
equals HF / e minus W minimum / e and
look at this stare at this equation
Einstein says and I am staring at it and
Einstein says mahes look this equation
resembles your equation for straight
line y = mx + C this means if you were
to plot a graph of the stopping voltage
versus the frequency for any material
for photo thetic experiment then we
should get a straight line which has a
slope of this number and The Intercept
of this number and what's weird over
here is that the slope has planks
constant in it since we already know the
value of e we can now do an
experiment calculate the slope and
figure out the value of Plank's constant
and Robert Milan is thinking what are
the chances that the value of the planks
constant which we get from a Photon
Theory trying to explain photo lric
effect is the same as the planks
constant gotten by Max plon was trying
to do a counting trick what are the
chances of that there is no chance that
is true and I am going to prove it to
you that that is not that's not
happening and that's why this experiment
this whole thing would be wrong that's
what Robert migan was thinking if you
think from his perspective yeah it kind
of makes sense back then what are the
chances that the value has to be same
but moreover since this slope is a
universal constant this means regardless
of what material I use what metal I use
the slope has to be exactly the same it
can have different intercepts but they
should all have the same slopes and
again milikan was like what's the
chances of that happening that's
definitely not going to happen no
chance okay he wasn't really evil but he
had a really good point the claims made
by the photoelectric equation was really
really bold and milhan just wanted to
disprove it so what happened well for
the next few years he meticulously does
this experiment slowly changing the
frequency calculating the stopping
voltage and drawing that graph and it
does all of that and finally publishes
his results what did he publish well a
direct photo electric determination of
Plank's constant he finds that the slope
of the graph of various Metals is
exactly the same it's a universal
constant and when he finds the value of
H he finds it to be exactly the same as
what PL found the writing was on the
wall Einstein and his Photon model had
won this is the actual photo of milon's
writing and now you can truly appreciate
how Monumental this piece of paper is it
validated Einstein's theory this was the
final nail in the coffin for the trigger
hypothesis and although World War I
started a little bit later and that
slowed things down but it was inevitable
in 1921 Einstein finally won his Nobel
Prize and he got it for the photo effect
why because he found something
revolutionary completely changed the way
he thought about light now relativity
was also awesome but nobody understood
relativity back then so it would be much
less controversial if you a it for the
photoelectric effect also Einstein won
this nebble prize the next year in 1922
why Leonard hit that guy anyways this is
how a small accidental Discovery by HRI
Herz catapulted into the discovery of
the quantum model by Albert Einstein but
when Einstein probably had no idea what
he had done he had completely shaken
physics because for a long time we
thought we knew what light was and now
we're back to square one is it a
particle is it a wave how do you
reconcile all of this together sure
today we know about the Dual nature of
light but it wasn't such a
straightforward jump What followed next
What followed over here is a lot of
chaos more confusion and even more
personal battles but it's even more
interesting I had a lot of fun making
this particular video and I'm super
curious to see would you liked it or not
if you did oh my got there is so much I
have to share when it comes to Quantum
Revolution all right but in the
meanwhile if you haven't already you can
check out my video on Heisenberg's
uncertainty principle which is linked
somewhere over here it's also completely
intuitive and I know you might probably
need a break over here but do check it
out and you feel like it all right
that's it see you