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  • - We've been treating light as a wave,

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

  • of oscillating electric and magnetic fields

  • that are traveling in some direction.

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

  • If you sent it through a small opening,

  • this electromagnetic radiation would spread out,

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

  • Or, if you let it overlap with itself,

  • if you had some wave in some region,

  • and it lined up perfectly

  • with some other electromagnetic wave,

  • you'd get constructive interference.

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

  • That's what waves do.

  • Why shouldn't we call electromagnetic radiation a wave?

  • And that's what everyone thought.

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

  • physicists discovered something shocking.

  • They discovered that light,

  • and all electromagnetic radiation,

  • can display particle-like behavior, too.

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

  • Waves can get localized.

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

  • well, that wave pulse is pretty much localized.

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

  • kind of look like a particle.

  • That's not really what we mean.

  • We mean something more dramatic.

  • We mean that light, what physicists discovered,

  • is that light and light particles

  • can only deposit certain amount of energy,

  • only discrete amounts of energy.

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

  • no less than that.

  • So this is why it's called quantum mechanics.

  • You've heard of a quantum leap.

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

  • And so what do we call these particles of light?

  • We call them photons.

  • How do we draw them?

  • That's a little trickier.

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

  • so we kind of split the difference sometimes.

  • Sometimes you'll see it like this,

  • where it's kind of like a wavy particle.

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

  • Basically, this is the problem.

  • This is the main problem with wave particle duality,

  • it's called.

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

  • can behave in a way that shows wavelike characteristics,

  • it can show particle-like characteristics,

  • there's no classical analog of this.

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

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

  • and a particle.

  • So it's impossible, basically,

  • to draw some sort of visual representation,

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

  • So we draw our photons like this.

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

  • if you had a detector sitting over here

  • that could measure the light energy that it receives

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

  • if that detector was sensitive enough,

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

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

  • You couldn't get in between.

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

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

  • three units of energy you could absorb,

  • if that was the amount of energy for that photon,

  • if these photons were carrying three units of energy,

  • you could either absorb no energy whatsoever

  • or you could absorb all three.

  • You can't absorb half of it.

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

  • You could either absorb the whole thing or nothing.

  • That's why it's quantum mechanics.

  • You get this discrete behavior of light

  • depositing all its energy in a particle-like way,

  • or nothing at all.

  • How much energy?

  • Well, we've got a formula for that.

  • The amount of energy in one photon

  • is determined by this formula.

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

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

  • and times f.

  • This is it.

  • It's a simple formula.

  • F is the frequency.

  • What is Planck's constant?

  • Well, Planck was basically the father of quantum mechanics.

  • Planck was the first one to figure out

  • what this constant was and to propose

  • that light can only deposit its energy in discrete amounts.

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

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

  • 10 to the negative 34th?

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

  • Times the frequency -- this is regular frequency.

  • So frequency, number of oscillations per second,

  • measured in hertz.

  • So now we can try to figure out,

  • why did physicists never discover this before?

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

  • that the energy of these photons are extremely small.

  • The graininess of this discrete amount of energy

  • that's getting deposited is so small

  • that it just looks smooth.

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

  • or at least it's very hard to tell.

  • So instead of just saying 'three units,'

  • let's get specific.

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

  • Well, the frequency of violet light is

  • 7.5 times 10 to the 14th hertz.

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

  • 6.626 times 10 to the negative 34th,

  • you'll get that the energy of one violet photon

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

  • Five times ten to the negative 19th,

  • that's extremely small.

  • That's hard to see.

  • That's hard to notice,

  • that energy's coming in this discrete amount.

  • It's like water.

  • I mean, water from your sink.

  • Water flowing out of your sink looks continuous.

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

  • and that you can only get one water molecule,

  • no water molecules, 10 water molecules,

  • discrete amounts of these water molecules,

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

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

  • The same is happening with this light.

  • This energy's extremely small.

  • Each violet photon has an extremely small amount of energy

  • that it contributes.

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

  • a baseball, a professional baseball player,

  • throwing a ball fast, you know,

  • it's about 100 joules of energy.

  • If you wanted to know how many of these photons,

  • how many of these violet photons would it take

  • to equal the energy of one baseball

  • thrown at major league speed?

  • It would take about two million trillion

  • of these photons to equal the energy

  • in a baseball that's thrown.

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

  • For all intents and purposes, for all we care,

  • at a macroscopic level, light's basically continuous.

  • It can deposit any energy whatsoever,

  • because the scale's so small here.

  • But if you look at it up close,

  • light can only deposit discrete amounts.

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

  • small amounts.

  • Light can deposit an enormous amount of energy,

  • but it does so in chunks.

  • So think about it this way ...

  • Let's get rid of all this.

  • Think about it this way:

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

  • how much energy it's absorbing,

  • and we'll graph it.

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

  • the amount of energy per time that it measures.

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

  • Now, you can absorb huge amounts of energy.

  • And on the detector, on a macroscopic scale,

  • it just might look like this.

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

  • You're absorbing more and more energy,

  • collecting more and more energy.

  • But what I'm saying is that, microscopically,

  • if you look at this,

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

  • You absorbed another one,

  • absorbed another one,

  • absorbed a bunch of them.

  • You keep absorbing a bunch of these photons.

  • You can build up a bunch of energy.

  • That's fine.

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

  • you have this step pattern

  • that's absorbing photons at a time,

  • certain numbers of them.

  • Maybe it absorbs three at one moment,

  • four at another moment.

  • But you can't absorb anything in between.

  • It can't be completely continuous.

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

  • of absorption of energy that, on a macroscopic scale,

  • looks smooth but on a microscopic scale is highlighted

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

  • described by this equation

  • that gives you the energy of individual photons of light.

- We've been treating light as a wave,

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B1 中級

光子エネルギー|物理プロセス|MCAT|カーンアカデミー (Photon Energy | Physical Processes | MCAT | Khan Academy)

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    許藝菊 に公開 2021 年 01 月 14 日
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