- If you shine particular kinds of light on certain metals,
electrons will be ejected.
We call this the photoelectric effect because light is photo
and electrons being ejected is electric.
And this was one of the key experiments
that actually helped us discover a completely
new model of light.
But how exactly you ask, well, let's find out.
What's interesting here is that this effect depends on
the color of light.
For example, if this metal was, say potassium, okay,
then if you shine blue light,
then we will get electrons being ejected,
photoeletric effect happens.
But if you were to shine red light on potassium,
we will not get photoeletric effect at all.
Regardless of how bright you make it.
Even if you were to make it blindingly bright,
we will not get photoeletric effect.
This is what puzzled physicists.
I mean, think about the model over here we have atoms
with electron clouds over here and the nucleus
at the center, okay?
When you shine light, the energy of the light
gets transferred to the electrons and they're able to escape
the clutches of the nucleus and go out.
But why can't that happen over here with red light?
Think about it, I'm shining bright light,
very high intensity, incredible amount of energy over here,
and yet electrons are not able to absorb it and get out.
Why does the photoeletric effect depend on the color?
That was a big question that didn't make any sense.
So what do we do over here?
Well, we do more careful experiments.
First, let's only look at the color
and then think about the brightness later, okay.
So what is color representing electromagnetic waves?
Remember that color basically depends on the wave
length of light.
For example, red color is the wavelength of light
could be somewhere around 650 nanometers.
What we find is that at 650 nanometers
we don't get any photoelectric effect for potassium.
We don't know why, but now what we can do is
let's reduce the wavelength and see what happens
if I keep reducing the wavelength, and let's say
I come to orange light of 600 nanometers,
see I've reduced it.
I still get no photoeletric effect, I don't know why,
but I'm just doing an experiment.
This is an observation, okay?
I keep reducing.
I keep reducing the wavelength until I hit 541 nanometers.
At this point, I now start seeing photoeletric effect,
and in this particular case, electrons are barely ejected
from the metal.
That's why I've drawn very tiny arrow marks over here,
they have hardly have any kinetic energy.
I just get some photoeletric effect
and then if I reduce it even further, that's where I get my
blue light at about say 500 nanometers.
These are rough numbers, okay?
At 500 nanometers, I now get photoelectric effect,
but the electrons coming out with even more energy.
What happens if I reduce it even further?
I find that electrons are coming out with even more energy.
So what's our observation over here?
We see that if we lower the wavelength, we get more energy
for the electrons coming out over here.
We can also talk in terms of frequency.
Remember, bigger the wavelength, smaller the frequency,
because if you have big wavelength,
there are less waves passing per second.
So this is low frequency and this is high over here.
So we can say when it comes to frequency,
more the frequency, more the energy of the electrons.
And you also have some kind of a cutoff over here, right?
So for example, if the wave length is above 541 nanometers
for potassium, for potassium, if it's above 5 41 nanometers,
no photoeletric effect, only below it,
we will get photoelectric effect.
And so every metal will have its own cutoff.
We call that the threshold wavelength,
or you can also say threshold frequency.
But the whole idea is if the wavelength is below
that threshold wavelength only then you get
photoelectric effect, if it's about you won't get it.
Different metals have different threshold wavelengths
and similarly different threshold frequencies.
So that's the effect of wavelength of frequency.
We see that the wavelength of the frequency controls
the energy with with the electrons come out
and that cannot be explained as to why.
Why does the wavelength of the frequency control it?
Why am I not getting photoeletric effect?
If it's about the threshold wavelength,
it doesn't make any sense.
But anyways, the next question could be for us,
how does the brightness affect this whole thing?
Does it have any effect?
The answer is yes.
Remember, brightness or the intensity of light is basically
how big the valleys and the peaks are, right?
So if you were to make the light brighter,
then it will look somewhat like this.
You can imagine it this way.
This is brighter light, okay?
Now what we find is that we get more electrons, okay?
It doesn't change the energy with these electrons come out.
See they're coming out with much the same energy as before,
but we now get more electrons.
Of course, if you're above the threshold wavelength,
you'll not get photo photoelectric effect at all,
regardless of the brightness.
It doesn't matter, okay?
So if you decrease the brightness or intensity,
you get less electrons.
If you increase the intensity, you get more electrons.
So intensity only controls the number of electrons,
but it's the wavelength of the frequency that controls
the energy with visual electrons come out,
it also controls whether we get photoeletric effect or not.
The big question was why the wave model just cannot explain
this because according to wave model,
you should get photoeletric effect for all colors
of light, right?
If you make light bright enough, electrons should be able
to absorb it and just get, you know just get admitted.
But that doesn't happen.
And this is why physicists back then were puzzled
and we were desperately in need of an answer for this.
So what did we do?
Well, to explain these observations, we came up with
a completely brand new model of light.
Instead of thinking of light as waves that carry
energy continuously and that can transfer
energy continuously, we thought maybe light
is made of discrete packets of energy, not waves,
but packets of energy, which we call photons.
And then light is being absorbed by say electrons.
You also absorb it as packets.
You'll either absorb no light or you'll absorb
one packet of light or two packets of light
and so on and so forth, nothing in between.
We call this discrete, which is exactly opposite
of what happens in wave model,
there you can absorb continuously.
Okay, so how does this explain the photoelectric effect,
the observations over here?
Well, let's see.
The key thing over here is that the energy of the photons
or the packets notice depends on the color.
If you're dealing with long wavelength
or low frequency light, then we have less energy
of the packet, the photons have less energy.
And if you're dealing with short wavelength
or high frequency light, you can see that the packets have
more energy.
So shorter the wavelength or more the frequency,
there is more energy in the packet.
There is a relationship between energy and the wavelength,
which we'll not get into.
But lemme just give you some rough numbers over here
because the numbers is gonna help us over here.
So here are some numbers.
So it turns out that if you consider red light
of 650 nanometers, the energy of the packet, the energy
of the photon is about 1.9 electron volt.
Yeah, maybe wondering, shouldn't we be measuring
energy in joules?
Well, joule turns out to be a big unit of energy.
So we use a smaller unit of energy,
which we call electron volts.
Don't worry too much about the units over here,
it's just the numbers.
You can see these packets have tiny energy,
but this packet has much bigger energy.
2.8 electron volts, you can see that, right.
Now for potassium it turns out,
if you want to pluck an electron,
if electron needs to be ejected, the minimum energy
that you need is about 2.3 electron volts.
This is for potassium.
Now is a great time for you to pause the video
and see if you can try and come up
with an explanation over here.
Alright, let's see.
The big idea over here is that if you want to knock off
an electron, I mean like you know,
make that electron escape, then a single photon
should have at least this much energy.
If the photons do not have at least this much energy,
then the electron will absorb it,
but it's not enough to escape,
and so it'll just reradiate it back.
And therefore, if you have consider red light,
it does not have a single photon,
does not carry enough energy.
And that's the reason why electrons are not
getting injected over here.
And that's why these lights are unable
to give you photoelectric effect.
Over here, we have just enough energy
for photoelectric effect and therefore electrons
barely make it out over here, because all of the energy
is used up in just releasing the electrons.
There's hardly any energy left over here,
so there'll be hardly moving.
But over here, notice you have more than the necessary
energy over here and therefore some residual energy is left.
And so electrons after coming out have some extra energy
remaining that goes out as kinetic energy.
And since this has even more energy,
each photon has even more energy while electrons now
eject with even more kinetic energy,
'cause there's more residual energy after getting ejected.
But what about the intensity?
Well, if you increase the intensity in this model,
we are increasing the number of photons, that's it.
Over here notice if a single photon does not have
enough energy, then I don't care how many photons you shine,
it's just not going to work.
That's why here I will still not get any,
you know, photoelectric effect.
But over here now I'm shining more number of photons,
so more electrons can absorb that energy
and therefore more electrons can escape per second.
And that's why I get more electrons over here,
putting it all together.
Since the wave of the frequency decides the energy
of an individual photon that decides the kinetic energy,
shorter the wavelength, stronger, more is the energy
of the photon and more is the kinetic energy.
If the wavelength is bigger and it's too big,
the energy of the photon is very tiny,
it'll not be able to knock off anything
and you'll not get any photoelectric effect.
And since intensity is basically the number of photons,
if you have more number of photons, you'll get more number
of electrons coming out.
But over here, it doesn't matter how many photons you shine,
and therefore it doesn't matter what the brightness is,
you will not get photoelectric effect.
Beautiful, isn't it?
So wait, does this mean that light is not a wave?
It's actually particles?
Well, not quiet.
You see certain phenomena of light like diffraction
or interference means that light must have wave properties
and certain other phenomenon like photoelectric effect,
black body radiation, scattering of light,
and other such effects makes us believe
that light must also have the particle nature,
the photon nature, which means a light must have
a dual nature, both particle and waves.
It's not that light sometimes behaves as waves
and sometimes wears as particle.
No, no, no.
Light has both wave and particle nature.
And if you're wondering, well, how does that make any sense?
How can something be both waves
and particles at the same time?
Well, unfortunately, there's no way to really visualize it
because in our macroscopic world,
we don't have any experience of things having both wave
and particle nature.
But this is one of the reasons why sometimes when we are
showing photons, we show it this way
with a tiny wave packet, but this doesn't mean
that the photons are wiggling up and down.
Okay, that's a misconception that I used to have.
It's not like that.
A better way to sort of think about this is that
light is not a wave in the traditional sense.
It's not a particle in the traditional sense,
it's a brand new object, which we don't have
experience within our daily life.
This object has both wave properties
and particle properties, and we call such an object,
a quantum object.
Now this sounds very theoretical, right?
But there are so many applications of the fact
that light is a quantum object.
Let me tell you one of them, okay?
Now, in photoelectric effect from light,
we get electrons ejected, right?
Now there's a very similar,
slightly different effect called (indistinct)
when you shine light, you can generate voltage.
We call such an effect, a photovoltaic effect.
Now, the way that works is we need to first create a crystal
in which there's an already inbuilt electric field.
It's possible to do that.
We'll not get too much details of how we build
such crystals, but using semiconductors,
we can build crystals like that.
We don't have to hook it up to any battery or anything.
It'll have an inbuilt electric field.
The crystals is built in such a way that one side
of the crystal has slightly different properties compared
to another side of the crystal.
And because of the difference in properties,
an electric field gets built up.
Now the important point is there are electrons everywhere,
but if you focus on this region, there are a lot of
electrons, but they're all bonded and they're not
free to move.
So even if there's an electric field pushing on them,
they cannot move because they're stuck in bonds.
You can imagine that this is like the sea of electrons.
They're all kind of fixed inside the crystal,
they cannot move.
But if we shine light in this region,
and if the light has the suitable frequency
or the suitable wavelength, then the electrons can absorb
that energy, but it won't get emitted.
Okay, that's the difference over here.
In photo effect, it gets emitted.
But here, instead of getting emitted,
it just gets enough energy to escape the bond.
And as a result, now it's free to move and therefore
it'll get accelerated to the left in this diagram,
because electric field to the right electrons are negatively
charged the experience of force in the opposite direction.
And as a result, it will now come to the left side
and it'll leave behind a gap.
Now, other electrons, under electrons, which are bonded,
can swoop into this gap, which makes the gap go
to the right, and then the other electrons can swoop into
this gap and so on and so forth.
So it kind of feels that this gap, this latency will move in
to the other side.
This way, a lot of electrons and a lot of vacancies
can be created.
And so look, if you can complete this circuit,
electrons would love to go from here to here
through that external circuit.
In other words, there is a voltage created.
And so what we have done is we have used the energy from
light to create voltage photovoltaic effect.
If you put a lot of these together, we create a solar panel.
That's how solar cells and solar panels work.
They work on the photovoltaic effect.
Whether you consider them on the roofs of the houses
or you consider the ones which are in the spacecraft,
they all use the same idea.
At the end of the day, we are using the fact
that light is a quantum object to harness the power
of light, which we get from the sun.