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- We've been treating light as a wave,

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and we've been drawing it with
this continuous wave pattern

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of oscillating electric
and magnetic fields

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that are traveling in some direction.

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And why shouldn't we treat it as a wave?

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If you sent it through a small opening,

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this electromagnetic
radiation would spread out,

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There'd be diffraction,
and that's what waves do.

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Or, if you let it overlap with itself,

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if you had some wave in some region,

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and it lined up perfectly

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with some other electromagnetic wave,

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you'd get constructive interference.

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If it was out of phase, you'd
get destructive interference.

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That's what waves do.

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Why shouldn't we call
electromagnetic radiation a wave?

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And that's what everyone thought.

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But, in the late 1800s and early 1900s,

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physicists discovered something shocking.

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They discovered that light,

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and all electromagnetic radiation,

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can display particle-like behavior, too.

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And I don't just mean localized
in some region of space.

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Waves can get localized.

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If you sent in some wave
here that was a wave pulse,

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well, that wave pulse is
pretty much localized.

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When it's traveling
through here, it's going to

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kind of look like a particle.

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That's not really what we mean.

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We mean something more dramatic.

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We mean that light, what
physicists discovered,

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is that light and light particles

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can only deposit certain amount of energy,

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only discrete amounts of energy.

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There's a certain chunk of
energy that light can deposit,

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no less than that.

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So this is why it's
called quantum mechanics.

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You've heard of a quantum leap.

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Quantum mechanics means a
discrete jump, no less than that.

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And so what do we call
these particles of light?

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We call them photons.

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How do we draw them?

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That's a little trickier.

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We know now light can behave
like a wave and a particle,

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so we kind of split the
difference sometimes.

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Sometimes you'll see it like this,

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where it's kind of like a wavy particle.

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So there's a photon,
here's another photon.

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Basically, this is the problem.

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This is the main problem
with wave particle duality,

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it's called.

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The fact that light, and
everything else, for that matter,

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can behave in a way that shows
wavelike characteristics,

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it can show particle-like characteristics,

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there's no classical analog of this.

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We can't envision in our minds
anything that we've ever seen

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that can do this, that can
both behave like a wave

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and a particle.

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So it's impossible, basically,

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to draw some sort of
visual representation,

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but, you know, it's always
good to draw something.

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So we draw our photons like this.

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And so, what I'm really saying here is,

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if you had a detector sitting over here

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that could measure the light
energy that it receives

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from some source of
light, what I'm saying is,

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if that detector was sensitive enough,

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you'd either get no
light energy or one jump,

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or no light energy or, whoop,
you absorbed another photon.

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You couldn't get in between.

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If the quantum jump was
three units of energy ...

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I don't want to give you a
specific unit yet, but, say,

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three units of energy you could absorb,

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if that was the amount of
energy for that photon,

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if these photons were carrying
three units of energy,

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you could either absorb
no energy whatsoever

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or you could absorb all three.

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You can't absorb half of it.

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You can't absorb one unit of
energy or two units of energy.

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You could either absorb
the whole thing or nothing.

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That's why it's quantum mechanics.

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You get this discrete behavior of light

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depositing all its energy
in a particle-like way,

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or nothing at all.

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How much energy?

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Well, we've got a formula for that.

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The amount of energy in one photon

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is determined by this formula.

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And the first thing in
it is Planck's constant.

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H is the letter we use
for Planck's constant,

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and times f.

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This is it.

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It's a simple formula.

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F is the frequency.

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What is Planck's constant?

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Well, Planck was basically the
father of quantum mechanics.

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Planck was the first one to figure out

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what this constant was and to propose

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that light can only deposit
its energy in discrete amounts.

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So Planck's constant is
extremely small; it's

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6.626 times 10 to the negative
34th joule times seconds.

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10 to the negative 34th?

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There aren't many other
numbers in physics that small.

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Times the frequency --
this is regular frequency.

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So frequency, number of
oscillations per second,

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measured in hertz.

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So now we can try to figure out,

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why did physicists never
discover this before?

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And the reason is, Planck's
constant is so small

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that the energy of these
photons are extremely small.

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The graininess of this
discrete amount of energy

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that's getting deposited is so small

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that it just looks smooth.

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You can't tell that
there's a smallest amount,

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or at least it's very hard to tell.

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So instead of just saying 'three units,'

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let's get specific.

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For violet light, what's the
energy of one violet photon?

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Well, the frequency of violet light is

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7.5 times 10 to the 14th hertz.

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So if you take that number
times this Planck's constant,

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6.626 times 10 to the negative 34th,

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you'll get that the energy
of one violet photon

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is about five times 10 to
the negative 19th joules.

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Five times ten to the negative 19th,

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that's extremely small.

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That's hard to see.

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That's hard to notice,

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that energy's coming in
this discrete amount.

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It's like water.

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I mean, water from your sink.

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Water flowing out of your
sink looks continuous.

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We know there's really discrete
water molecules in there,

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and that you can only
get one water molecule,

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no water molecules, 10 water molecules,

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discrete amounts of these water molecules,

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but there's so many of
them and they're so small,

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it's hard to tell that it's
not just completely continuous.

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The same is happening with this light.

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This energy's extremely small.

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Each violet photon has an
extremely small amount of energy

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that it contributes.

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In fact, if you wanted
to know how small it is,

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a baseball, a professional
baseball player,

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throwing a ball fast, you know,

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it's about 100 joules of energy.

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If you wanted to know how
many of these photons,

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how many of these violet
photons would it take

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to equal the energy of one baseball

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thrown at major league speed?

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It would take about two million trillion

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of these photons to equal the energy

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in a baseball that's thrown.

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That's why we don't see
this on a macroscopic scale.

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For all intents and
purposes, for all we care,

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at a macroscopic level,
light's basically continuous.

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It can deposit any energy whatsoever,

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because the scale's so small here.

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But if you look at it up close,

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light can only deposit discrete amounts.

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Now, I don't mean that
light can only deposit

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small amounts.

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Light can deposit an
enormous amount of energy,

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but it does so in chunks.

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So think about it this way ...

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Let's get rid of all this.

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Think about it this way:

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let's say you had a detector
that's going to register

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how much energy it's absorbing,

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and we'll graph it.

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We'll graph what this
detector's going to measure,

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the amount of energy per
time that it measures.

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So we'll get the amount
of energy per time.

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Now, you can absorb
huge amounts of energy.

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And on the detector,
on a macroscopic scale,

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it just might look like this.

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You know, you're getting
more and more light energy.

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You're absorbing more and more energy,

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collecting more and more energy.

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But what I'm saying is
that, microscopically,

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if you look at this,

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what's happening is, you've
absorbed one photon here.

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You absorbed another one,

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absorbed another one,

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absorbed a bunch of them.

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You keep absorbing a
bunch of these photons.

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You can build up a bunch of energy.

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That's fine.

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It's just if you looked
at it close enough,

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you have this step pattern

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that's absorbing photons at a time,

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certain numbers of them.

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Maybe it absorbs three at one moment,

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four at another moment.

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But you can't absorb anything in between.

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It can't be completely continuous.

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It has to be a discrete
all-or-nothing moment

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of absorption of energy
that, on a macroscopic scale,

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looks smooth but on a
microscopic scale is highlighted

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by the fact that light energy
is coming in discrete chunks,

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described by this equation

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that gives you the energy of
individual photons of light.