The photoelectric and photovoltaic effects | Physics | Khan Academy

- 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.