- The photoelectric effect is taught
in nearly every physics class,
but we often forget to tell why it's important.
In my mind, it's important,
because Einstein's photoelectric effect equation
was based on a revolutionary new view of light.
Now I use the word revolutionary,
because Einstein used that exact same term.
Einstein wrote his friend that the paper
he was working on deals with, quote,
"The energy properties of light and is very revolutionary."
Why did Einstein think of this?
Why did it kick start the quantum mechanics revolution?
And how did he do it in the middle of some personal drama,
much of his own making?
Well I'll tell you, ready?
Let's go.
♪ Electricity, electricity ♪
Let's begin in April of 1901
when 22-year-old Albert Einstein read a paper
by a conservative German physicist named Max Planck
on how certain heated objects radiate light.
Planck included this wild math trick,
where he assumed that light was created in little bundles
with an energy equal to a constant h,
now called Planck's constant times its frequency.
Note, Planck use the Greek letter nu,
which looks like v for frequency.
Sorry for the confusion.
Einstein was not impressed
and wrote his girlfriend, Mileva,
quote, "About Max Planck studies on radiation,
"misgivings of a fundamental nature have arisen in my mind,
"so I'm reading his article with mixed feelings."
Seven days later, Einstein was still complaining
that Planck's idea that the thing that creates light
are limited to how they can vibrate was quote,
"An assumption I can't really warm to,
"and because of Planck," quote,
"My views on the nature of radiation
"have again sunk back into a sea of haziness."
In May, Einstein read a paper much more to his liking.
A former professor of Mileva's, named Phillip Lenard,
had written about an experiment
that proved that shining ultraviolet light on a metal plate
would cause electrons to fly off the plate.
This was only a few years after the discovery
that something called cathode rays in vacuum tubes
were actually beams of electrons.
This phenomenon quickly became known as
the photoelectric effect,
photo for light,
and electric for electric.
Einstein wrote Mileva, "My dear kitten,
"I have just read a wonderful paper by Lenard
"on the generation of cathode rays by ultraviolet light.
"Under the influence of this beautiful piece
"I'm filled with such happiness and joy
"that I absolutely must share it with you."
There's no hint in this letter that Einstein saw
any connection between the photoelectric effect
and Planck's energy bundles.
However, Albert and Mileva had bigger problems.
Mileva was pregnant with Einstein's illegitimate child.
The letter quickly changed from science to comfort.
"Be happy and don't fret darling," Albert added.
"I won't leave you and we'll bring everything
"to a happy conclusion.
"You just have to be patient."
Mileva had met Albert when she was studying physics
at Zurich Polytechnic,
and she was one of only 20 women
in all of Germany and Switzerland studying science or math.
Three years older than Einstein,
Mileva had come from Serbia to become a doctor,
but had switched to physics.
Her grades were fantastic until she fell for Albert.
Then well, not so fantastic.
By 1901, Mileva had failed her exams twice,
was pregnant and went home to confess
her troubles to her parents.
Meanwhile, Einstein couldn't find a job
mostly due to his delight in annoying
all people in authority and also due
to a small dose of antisemitism
and refused to marry her without one.
Mileva wrote a friend, "Albert has not yet found a job,
"and you know that my darling has a very wicked tongue.
"And on top of it, he's a Jew."
"Don't worry," Albert wrote.
"I will look immediately for a position.
"No matter how humble, my scientific goals
"and personal vanity will not hinder me
"from me accepting the most subordinate role.
"The moment I have obtained such a position,
"I'll marry you."
But Albert couldn't find a job
or a job he would accept.
And Mileva had a daughter, Lieserl in secret
in the beginning of 1902.
By June of 1902,
he got a job as a patent clerk
in Bern, Switzerland.
And in January, the following year,
Mileva went to Bern, leaving the baby
with her parents and married Albert.
In August, Mileva went back to Serbia
and found that she was pregnant again,
and that their daughter had scarlet fever.
Albert was mildly concerned about Lieserl,
but pleased with the pregnancy
and told Mileva that, quote,
"I'm happy you get a new Lieserl."
Oy, Einstein, darling,
you can't just replace babies.
He also told her to hurry home as, quote,
"A good little wife shouldn't leave
"her husband alone too long."
The place was getting messy.
But don't worry, quote,
"You'll be able to clean up in short order."
Sigh.
Anyway, it seemed like either Lieserl
was adopted away or died of scarlet fever
as that's the last that anyone heard of her.
In May of 1904, Mileva did not get another Lieserl
as she had a boy, Hans Albert.
Despite what you might have read or heard,
Einstein enjoyed his job as a patent clerk,
finding it, quote-unquote,
"Uncommonly diversified and pretty easy."
In fact, Einstein later recalled that he was able to,
quote "Do a full day's work only one or two hours.
"And the remaining part of the day,
"I would work on my own idea,
"but when anyone came by, I would cram my notes
"into my desk drawer and pretend to work on my office work."
In this way, the Einsteins kept up with the latest research
and Albert, at least, has a lot of time
to think of new ideas.
At around this time,
Einstein read that Phillip Lenard
had conducted a new experiment
on how ultraviolet light creates electrons
and the photoelectric effect
that Einstein called pioneering.
See Lenard had the very clever idea
of adding an extra voltage the wrong way,
which he called a counter voltage.
so that the electrons freed by the ultraviolet light
would be pushed back towards the irradiated plate.
As Lenard said, quote,
"In the same way as a stone thrown upward
"falls back to the ground."
Lenard knew that the voltage to stop the electrons
from reaching the other plate could tell him
about the top energy of the freed electrons.
Then Lenard found something truly startling.
The top speed of the freed electrons
was unaffected by the intensity of the light.
Lenard decided that the energy of the electrons
did not come from the light,
but instead the light acted like a trigger
to release the inherent energy of the atoms
like quote, "The fuse in firing a loaded gun."
This triggering theory seemed like a good solution.
And even as late as it was 1909,
it was written as one of the generally
accepted truths of physics.
However in 1904 or so, Einstein had a quirky thought.
What if the energy of the electrons did come from the light,
but the light didn't act like a typical wave.
See, for a typical wave,
like a sound wave or a water wave,
the higher the intensity,
the greater the energy.
So more intensity ultraviolet light
should lead to higher energy freed electrons,
but that's not what Lenard had observed.
However, Einstein remembered Planck's paper
were Planck assumed that light was created
in little energy packets with an energy
that depended on the frequency of light.
Now, Planck just did this as a math trick,
but Einstein started thinking,
what if it was literally true?
What if light was composed of tiny little particles
that could only be created, moved or absorbed as a whole.
Einstein called these particles quanta,
which is a word for units that are indivisible,
which is why this branch of physics
is called quantum mechanics,
quantum being the singular of quanta.
In 1905, Einstein wrote, quote,
"The assumption to be contemplated here
"is that when a light ray is spreading from a point,
"the energy is not distributed continuously
"over ever-increasing spaces,
"but consists of finite number of energy quanta
"that are localized in points in space,
"move without dividing,
"and can be absorbed or generated only as a whole."
This has been called, quote,
"The most revolutionary sentence written
"by a physicist of the 20th century."
Before this paper,
items were either waves or particles.
This was the first proposal that they could be both.
Light has frequency and creates
interference patterns like a wave,
but is composed of little energy packets
that can not be split in two
just like you can't split an electron in two
like a particle.
Later, Einstein described it, quote,
"As a kind of fusion of the wave
"and particle theories of light."
If this makes your brain hurt,
you are not alone.
Even Einstein was never really comfortable with it.
And in 1951, he wrote his friend Besso, quote,
"All these 50 years of pondering
"have not brought me any closer to answering the question,
"What are light quanta?"
Despite not really getting what light quanta are,
Einstein found that this counterintuitive idea
of treating waves like a bunch of particles
could explain all sorts of phenomenon,
including the photoelectric effect.
Einstein decided that in the photoelectric effect,
electrons on the plate are stuck
to the metal by electric forces.
And it can only be removed by giving them
a certain amount of energy,
which they get from absorbing the energy
of the incoming light.
Einstein then postulated that the light came
in a little energy packets,
where the brighter the light,
the more packets,
that the individual light packets,
now called photons,
all have the same energy.
If the frequency is too low,
the photons of light have less energy
than the binding energy
of the surface electrons to the metal,
and no electrons will be freed from the plate
no matter how many packets you throw at the plate.
This is why you need relatively high frequency light
to create the photoelectric effect,
also why UV light creates sunburn
and visible light does not.
Above this threshold,
the more intense the light,
the more electrons are freed,
but the electrons all have the same maximum energy,
the energy of the photon of light
minus the minimum energy to free the electron.
According to Planck's equation,
the energy of light
is Planck's constant h times the frequency.
And according to Einstein,
the minimum energy to free the metal
is a constant of the metal.
Einstein also knew from energy conservation
that the voltage to stop all the electrons
from moving to the other plate
times the charge on the electron
equals the maximum kinetic energy of the electron.
Thus, Einstein created the equation
for the photoelectric effect.
Interesting, in this equation, for any metal,
there are only two variables in this equation:
the stopping voltage and the frequency.
Therefore, Einstein theorized that if you graph
the stopping voltage versus the frequency of light,
you should get a straight line
no matter what material you use for the plate.
Even more impressively,
the slope of that line can give you Planck's constant h.
Einstein found an experimental way
to prove that light comes in little energy packets
with an equation equal to a constant
times the frequency of light,
and even found a new way
to find a universal constant of nature.
You'd think an experimentalist would have jumped on this.
However, the experiment was ridiculously difficult to do,
and no one was much interested
in what an annoying patent clerk third class
had to say on the issue anyway.
In fact, it took nine years for any experimentalist
to successfully do this experiment,
at which point, the scientist who did it,
Robert Millikan of oil drop fame,
found his results were quote,
"A complete success of the Einstein equation."
Although Millikan continued to reject
what he called the bold,
not to say reckless, theory behind it.
A comment about Mileva in all this.
It is clear that she helped her husband
with his papers at this time,
as sometimes both Albert and Mileva called it
"Our work" in letters to each other.
And Albert had also told Mileva's friends, quote,
"I need my wife.
"She's solves all the mathematical problems for me."
There are no mentions, however,
of Albert attributing any ideas to his wife.
Is this because she made no contributions?
Or, was it because he was kind of sexist
and he considered her basically his student,
as he affectionately called her,
and didn't feel like her contributions
needed to be mentioned.
This was how it was portrayed
in the mostly excellent 2017 television show, "Genius."
- You thanked Michele.
- [Albert] Of course, I thanked him.
- But I've helped you with so many papers,
but you never thought to put my name in any of them.
- It never occurred to me.
- It also should not be discounted
that Einstein was trying to be recognized in 1900s Germany,
not a place where one would be respected
for mentioning the contribution of one's wife.
Anyway, as I said in the beginning,
Einstein thought correctly
that this paper was very revolutionary
and therefore was not surprised to find
it didn't get very far at first.
He had higher hopes for his next paper
on Brownian motion to prove the existence of atoms.
And it did get published,
but it didn't go very far.
His next paper was on a strange idea called relativity.
That was the paper that slowly got the attention
of some prominent scientists like Lorentz and even Planck.
Strangely enough, Planck loved relativity instantly
and fought against quantum mechanics
that Planck himself accidentally initiated.
In addition, that same year,
Einstein published addendum to his relativity paper.
An amusing and seductive thought that the, quote,
"Relativity principle together with Maxwell's equations
"requires that mass be a direct measure
"of the energy contained in a body."
In other words, Einstein published that E equals mc-squared.
One would think that any of these papers
would have catapulted Einstein into scientific stardom.
After all that year, 1905,
is considered so influential from him
that it is called a miracle year in science
like 1666 for Newton.
However, it was a really slow burn.
It took him until 1909, four years later,
for him to even get an associate professor position.
And it wasn't until the next year, 1910,
that he became famous among scientists,
not for relativity,
but for quantum mechanics.
In fact, Einstein's 1911 talk
at the first Solvay conference,
The first international science conference,
was on quantum mechanics.
How did Einstein become famous?
And what does that have to do
with the third law of thermodynamics
and absolute zero temperature?
Well, I'll tell you
next time on the "Lightening Tamers."
Thanks for watching my video.
I hope I didn't mess it up too much.
And I know I cannot say Mileva's name correctly.
I think I said it 12 different ways.
I forgive myself.
I'm sorry, pronunciation is not my thing.
So anyway, if you wanna know more about Lenard,
I have a video about him.
If you wanna know more about Planck,
I have a video about him.
I have a lot of videos.
So please share it around.
Give me a thumbs up, give me a comment,
complaining about my pronunciation or anything else.
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and to Atoms and Sporks who helped me out with this.
If you wanna become a patron,
you can go down below.
If you wanna know more about Atoms and Sporks' website,
which describes modern physics very well,
I also have a link down below.
Okay, have a nice day.
(upbeat jazz music)