If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains ***.kastatic.org** and ***.kasandbox.org** are unblocked.

Main content

Current time:0:00Total duration:9:18

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 travelling 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 - 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 amounts 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 wave-like characteristics it can show particle light 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 could 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 that if that detector was sensitive enough you'd either get no light energy or one jump or no light energy or what 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's a simple formula f is the frequency what is Planck's constant will plank plank 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 that is extremely small it's 6.6 to 6 times 10 to the negative 34 joule times seconds 10 to the negative 34 there aren't many other numbers and 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 so you had a violet how what's the 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 six point six to six times ten to the negative 34 you'll get that the energy of one violet photon is about five times 10 to the negative 19 joules five times 10 to the negative 19th that's extremely small that's hard to see that's hard to notice that energy is 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 ten 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 is extremely small each violet photon has an extremely small amount of energy that it contributes in fact if you wanted to know how small is a baseball a professional baseball player throwing a ball fast you know it's about a hundred 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 animal 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 basically continuous they can deposit any energy whatsoever because the scale is so small here but if you look at it up close like and 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 detectors 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 it on the detector on a macroscopic scale it just might look like this you know you're getting more and more light energy absorbing more and more energy and collecting more and more energy but what I'm saying is it 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 or 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 no thing moment of absorption of energy that on a macroscopic scale looks smooth but on a microscopic scale as 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