For more information, please see full course syllabus of AP Chemistry

For more information, please see full course syllabus of AP Chemistry

### Light

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

- Intro 0:00
- Light 2:14
- Introduction to Light
- Frequency, Speed, and Wavelength of Waves
- Units and Equations
- Electromagnetic Spectrum
- Example 1: Calculate the Frequency
- E = hν
- Example 2: Increment of Energy
- Photon Energy of Light
- Wave and Particle
- Example 3: Wavelength of an Electron

### AP Chemistry Online Prep Course

### Transcription: Light

*Hello, and welcome back to Educator.com, and welcome back to AP Chemistry.*0000

*Today, we are going to begin our discussion of elementary quantum mechanics.*0005

*We are going to begin with a discussion of light and the general behavior of light--things like frequency, wavelength...some of it is properties.*0011

*This is, for me personally, really an extraordinary, extraordinary topic.*0021

*For those of you that are chemistry majors and physics majors, you have some really, really amazing things to look forward to, because when you take your course in quantum mechanics in the next couple of years (probably in your junior or senior year), it really is an extraordinary, extraordinary class.*0027

*It's very, very unusual in the sense that particles--they just don't behave the way you expect them to.*0044

*It is all very, very counterintuitive, and yet it's all very, very beautiful.*0054

*It begins with our discussion of probably the most ubiquitous thing in our life, which is light.*0058

*Now, I have to warn you: this particular topic and the next couple of chapters that we are going to discuss (which is going to consist of bonding, basic bonding theory, other bonding theory)...we run the risk of actually saying too much when we discuss some of these things.*0066

*And it is interesting: these are some of those topics that, by saying too much about them, you actually end up causing some confusion.*0087

*Now, I am hoping that we don't actually fall into that trap, so if it seems like some of the things that I'm saying are just sort of...I'm dropping them, dropping them, dropping them, there is a saying in a lot of science: "Much of science seems to be difficult to understand; it's just a question of getting used to it."*0094

*So, if there is something that you can't completely wrap your mind around, don't worry about it: the more we discuss it, the more problems we do--you will actually get used to it, and you will sort of become accustomed to it, and understand it sort of from the back end, as opposed to from the front end.*0111

*That is just the nature of some of the things that we are going to be discussing.*0127

*With that, let's just sort of jump in and see what happens.*0130

*OK, so a fancy term for light: you all know what light is, at least from your daily experience with it; a fancy term for light is electromagnetic radiation.*0134

*Let me write that down: A fancy term for light is electromagnetic radiation.*0146

*So, yes, light is a form of radiation; so when you are exposed to light, you are being exposed to a type of radiation.*0165

*Now, electromagnetic: well, that is because electricity and magnetism are actually two sides of the same phenomenon.*0173

*Two sides of a coin: electricity and magnetism are not separate things--they are actually different aspects of the same force, the electromagnetic force.*0180

*Light is energy; OK, energy travels in waves.*0192

*Any time you see, let's say, a wave on an ocean--what you are looking at is energy that is being transmitted through the medium of water.*0208

*That is what it is: if there were no energy being transmitted through the water, the water would just be like a swimming pool.*0220

*But, there is energy being transmitted through that water, and that energy takes the form of a wave.*0226

*Well, electromagnetic radiation is a form of energy; light is energy, and it travels in waves.*0231

*These EM waves have different energies, and in a minute, I will actually draw out the electromagnetic spectrum, so that you can see the different types of light that make up this thing that we call electromagnetic radiation.*0241

*These EM waves have different energies, but they all travel at the same speed, which happens to be--well--the speed of light...because that is what light is: light is electromagnetic radiation.*0265

*The symbol is c, and it is equal to 2.99x10 ^{8} meters per second (or 186 thousand miles per second)--very, very, very fast.*0287

*OK, now, waves have 3 things: they have speed; they have something called frequency; and they have something called wavelength.*0303

*Wavelength is pretty descriptive--it's the length of the wave.*0321

*Frequency--a little bit less descriptive, but we will define what it means in just a second.*0324

*Speed, we have already dealt with; it is the speed of light; so c is the speed of light.*0330

*Every type of electromagnetic radiation travels at the same speed: radio waves, x-rays, gamma rays, ultraviolet light, infrared...all of these things...microwaves...they all travel at the same speed.*0338

*It is their frequency and wavelength that distinguishes the different types of light.*0350

*Visible light, the light that we see, is a very narrow band, a very small part of the electromagnetic spectrum; it travels at 186,000 miles per second, 3 times 10 ^{8} meters per second.*0355

*OK, so let's draw out some waves here: so let's draw out a broad wave, and let's draw out something that is a little less broad, something like that.*0367

*OK...actually, I'm going to carry this wave out a little bit further.*0377

*A wavelength is exactly that: it is the length of one cycle: top of the wave, bottom, top of the wave--it's repeating--top of the wave, bottom, top of the wave; top of the wave, bottom, top of the wave.*0381

*This thing is called the wavelength, and its symbol is lambda--the Greek letter lambda--and that is the standard symbol for wavelength.*0399

*It is the length of a wave, usually from crest to crest; I mean, you can measure it any way you want, but the crest to crest is usually a good reference point.*0413

*You can go trough to trough--that is fine--but it's just the length of one wave.*0423

*OK, so let's see here: let me actually redraw these: so we have one, and then there we go; so let's do this again.*0429

*This is lambda; that is wavelength; and this is another lambda--this is another wavelength; so notice that this wavelength is actually smaller.*0445

*Well, so lambda is equal to the wavelength, and it has units of length.*0457

*It's exactly what it sounds like: sometimes it's expressed in meters; sometimes it's expressed in nanometers, centimeters...whatever the problem at hand seems to require.*0470

*Because we will be using mostly 3x10 ^{8} meters per second (it's the speed of light), we will be using the unit of length as a meter; but the problem might ask it in nanometers; you just have to learn to convert it.*0480

*So again, we have to watch our units in all of science, but particularly in quantum mechanics.*0494

*A frequency...well, I'm going to make this symbol: this is a symbol, the Greek letter nu (the Greek letter for n).*0500

*It looks like a v; it is not a v; my nus are probably a little different than most other people's nus--most people just sort of do it that way.*0509

*I don't like that: the reason is because I don't like to confuse it with velocity.*0516

*It happens sometimes, because velocity shows up--the letter v does show up in quantum mechanics; so it can be a little bit confusing if you are not careful.*0521

*This is called the frequency, and I will explain what it is in just a second: it is the number of cycles per second.*0531

*A cycle is just a repetition.*0542

*The unit of frequency is called a hertz, and its unit is per second, or usually, s ^{-1}.*0550

*It is actually cycles per second; you can think, if you will, for those of you that are familiar with trigonometry, radians per second--because again, remember, a radian is just going around a certain number of times.*0561

*You are going through a cycle; but this cycles part, we usually don't put in there; we just say "per second," so 13 hertz is 13 cycles per second.*0571

*That means, in every 1 second, the wave goes through that many cycles.*0582

*Well, a cycle is exactly what you think it is: it is the periodic motion of the wave.*0586

*So, in this case, from here to here is one cycle.*0590

*It is 1, 2, 3, 4, 5; so let's say, in this case, if this were a time length of 1 second, this would be like 1 cycle; this would be like 1, 2, 3, 4, 5; so this would be 1 hertz and 5 hertz.*0600

*That is the thing: the wavelength is the length of a wave from crest to crest of the wave; the frequency is the number of those waves--number of cycles that actually happen per second.*0617

*OK, so now we have a relationship between these two--a very, very important relationship.*0633

*The speed of light is equal to the wavelength, times the frequency of the particular type of light that we are talking about.*0639

*Another way of expressing this is: equals c over lambda.*0647

*It doesn't matter which one you actually use; you are going to be using it in both forms; I tend to remember it (I personally) as this: Nu equals c over lambda.*0653

*Notice that nu and lambda (frequency and wavelength) have an inverse relationship.*0662

*In other words, if the wavelength gets longer--gets bigger--the frequency gets smaller.*0685

*As the wavelength gets smaller--shorter--the frequency increases.*0696

*You can see that from the picture: this is a long wavelength, small frequency; this is a very short wavelength, high frequency.*0704

*Per second: one wave passes per second; in one second, five or six waves pass per second.*0717

*The wave is smaller here; the frequency is greater.*0725

*That is the relationship: so, a profoundly important relationship here.*0729

*Now, let's go ahead and actually draw out the electromagnetic spectrum and see what it is that we are looking at.*0734

*I'm going to do it this way--that is that, and I have this over here...this is going to be wavelength, and I'm going to use units of meters; and this is going to be frequency, and it is going to be in units of hertz (or per second).*0742

*This is the EM spectrum; OK, so let's go ahead and do 10, 10 ^{-2}, 10^{-4}, 10^{-6}, 10^{-8}, 10^{-10}, 10^{-12}...all right, I guess we can stop there.*0760

*Let's go here: 3x10 ^{8}, 3x10^{10}, 3x10^{12}, 3x10^{14}, 3x10^{16}, 3x10^{18}, and 3x10^{20}.*0795

*Radio waves are somewhere around here.*0829

*Radio waves have a wavelength of about 10 meters, so each wave of a radio wave is about 10 meters long.*0832

*Its frequency is about 3x10 ^{8} hertz--not bad.*0840

*TV is somewhere around there--a little bit shorter; microwaves--microwave radiation is on the order of about 10 ^{-3}--that is how long it is in terms of meters, so .001 meter--that is how long a microwave is.*0850

*And its corresponding frequency is just about there, so 10 ^{-4}; here we are talking about infrared radiation, and let's see: let's go over here to about 10^{8}; we have ultraviolet radiation.*0875

*We have x-rays just about here, and we have gamma rays just about here.*0895

*So you notice: radio waves, microwaves, infrared, ultraviolet, x-ray, gamma ray: the wavelength is getting shorter.*0903

*As the wavelength gets shorter, the frequency gets higher; that is the inverse relationship: notice--short wavelength: the more energy that wave has, the more powerful it is, the more damage it will do.*0911

*So, shorter wavelength--more damage; higher frequency--more damage.*0928

*OK, visible light fall in this range, just about here.*0936

*I'm going to magnify this: somewhere in the 10 ^{-6}, 10^{-7}...so I'm going to go ahead and expand that.*0944

*What you have is 1, 2, 3, 4, 5, 6; 1, 2, 3, 4, 5, 6; red, orange, yellow, green, blue, indigo, and violet.*0956

*Here, you are looking at about 7x10 ^{-7} meters and 4x10^{-7} meters.*0970

*So, all of this electromagnetic radiation--all of this is light.*0982

*This little region in here, between waves that have a wavelength of about 4x10 ^{-7} meters all the way up to 7x10^{-7} meters--that is the light that we see; that is visible light.*0987

*It is a form of radiation; it is part of the electromagnetic spectrum; in other words, the light that we see is an electromagnetic phenomenon.*1003

*It travels in waves, and this is the range of wavelengths for these.*1011

*Red is about 7x10 ^{-7} meters; violet light, 4x10^{-7} meters.*1016

*Smaller wavelength for higher frequency; it's more powerful.*1024

*Ultraviolet--that is what gives you the sunburn; so the radiation that we experience from the sun is actually in this range.*1029

*Ultraviolet and infrared: infrared is heat; when we feel heat, we are actually feeling infrared radiation--radiation that is electromagnetic in nature, and the wavelength of that light is on the order of 10 ^{-5}, 10^{-6}, things like that.*1038

*That is all that is going on here: this is the standard electromagnetic spectrum.*1055

*OK, so let's do an example...yes, I guess we can do it on this page: not a problem.*1060

*Example: OK, the red colors you see in fireworks is due (oops, hey, look at that--we have our crazy lines back again! OK) to the emission of light at around 650 nanometers (so here we gave it to you in nanometers) from strontium ions whose electrons have been excited to higher energies, then have fallen back down to lower energies to release the excess as light.*1068

*OK, so any time you see a firework, and you see the red part of the firework (whatever that happens to be), what you are seeing is a strontium ion whose electrons are...basically, we explode things; and when we explode them, that energy--the ions--it actually kicks up the electrons in the strontium ion to a higher energy level.*1171

*Well, it doesn't like being up at the higher energy level; it falls back down automatically--it's a spontaneous process for it to fall back down.*1195

*When it does that, that excess energy that it absorbed to rise--it actually gives it off as light; and the wavelength of that light is 650 nanometers, which happens to fall in the red range.*1201

*There you go; OK, so let's see: Calculate the frequency--that is our...*1219

*Well, this is nice and easy: so we said that nu equals c over lambda; it equals 2.99x10 ^{8} meters per second, over 650 nanometers (nano- is 10^{-9}, so we can just write 650x10^{-9} meters).*1236

*When we do this, we get: a frequency is equal to...see, watch: meter cancels with meter; you end up with: on the bottom, it's 4.6x10 ^{14}s, which is 4.6x10^{14} hertz: that is the frequency of the red light that you see in fireworks.*1263

*OK, so now, let's move on to our next phase; and again, we are just going to sort of throw out some ideas.*1291

*We are not going to get into too much explanation about it; a lot of the explanation will sort of come in the process of our discussion.*1300

*A lot of the really detailed explanation will come in courses that you take later on; so we are going to, in some sense, ask you to take this stuff on faith.*1307

*And it has actually worked out pretty well, because we enjoy the life that we enjoy around us: that has confirmed the fact that this stuff actually is true.*1315

*OK, so energy can be gained or lost in discrete units only, and that unit is hν, where h is the Planck constant (and that constant is 6.626...this has been determined experimentally...times 10 ^{-34} Joule-seconds; not Joules per second--Joule-seconds).*1323

*OK, and nu, as you know, is the frequency of the light of that energy; that is the frequency of electromagnetic radiation emitted or absorbed.*1382

*Emitted...I can't remember whether it's 2 t's or 1; oh, well--it doesn't matter.*1406

*OK, emitted or absorbed: so energy...we think of energy rising continuously or dropping continuously; as it turns out, it doesn't happen continuously.*1412

*It is very, very small, and it seems continuous, but energy moves in a stair-step fashion.*1423

*It doesn't go like this: it is not continuous--it is discrete; when energy rises or falls, it makes little jumps.*1435

*So, you have a certain amount of energy; the next amount of energy that you can have is actually a full unit up; or the amount of energy you can lose is a full unit down.*1445

*There are no in-between; it is discrete; it is not complete--it is not a continuum, like the real number line.*1459

*The real number line--if you were to put your finger anywhere on the real number line, you will always hit a real number.*1467

*As it turns out, energy is not like that; it is very, very discrete quantities; energy goes up in discrete units.*1472

*Now, this unit is very small; it is based on the particular frequency and this Planck constant, which is 10 ^{-34}; so it's true--from our perspective, it is so tiny that it actually looks continuous.*1479

*But it is not continuous; it is discrete, and that is what is important.*1495

*OK, so once again, let me rewrite that: the energy is Planck's constant, times the particular frequency--very, very important.*1500

*OK, so let's do another example: let's go to Example 2.*1512

*OK, for the previous strontium salts (or strontium ions, I should say...well, salts, because the actual material that we use in fireworks are the salts, but salts are made of ions, so...let me write this out a little bit better; I think I'm getting a little loose here), what is the increment of energy at that frequency?*1521

*OK, so we calculated a frequency of 4.6x10 ^{14} hertz (so I will write inverse seconds, hertz).*1578

*We want to know what kind of energy is associated with that red color in the fireworks.*1591

*Well, the increment of energy is equal to h times the frequency, which is equal to 6.626x10 ^{-34} Joule-seconds, times 4.6x10^{14} inverse seconds.*1598

*Second, second, inverse second, and inverse second cancel, leaving me with Joules--energy!*1626

*That increment of energy--that is why I have this Δ; when we speak about an increment, I am just taking (in this particular case) the initial as 0.*1633

*It is equal to 3.05x10 ^{-19} Joules.*1645

*Now, that is not a lot, but it is still a particular increment: so, in other words, if I have a unit of energy...so let's say I'm a ground level of energy--let's just call it a base level--the next rise in energy is 3.05x10 ^{-19}.*1656

*If I add a little bit more energy to that, it is not going to go to 4.06, 5.1...it is going to go to 6.1x10 ^{-19}.*1673

*And then, it is going to go to 9.15x10 ^{-19}; any jump that it makes in energy is going to happen according to this increment; that is it--nothing in between.*1684

*And, if that energy level is not available to it, guess what: it is not going to make that jump; that is the whole idea--that these are very, very specific jumps, based on the particular frequency of light.*1696

*That is pretty extraordinary: you think that, OK, let's say if I'm at 3.05x10 ^{-19} Joules of energy, can I add just a little bit more heat to make it go to 4?*1707

*The answer is no, you can't, because it won't do that; it will stay at 3.05.*1718

*And then, when it reaches its threshold, it will jump to 6.10, and then 9.15, and then 12.20; that is the idea.*1725

*It jumps in discrete units.*1734

*OK, so let's see what else we have here: so let's play around with some of these equations that we have introduced, and one that you actually are familiar with, that we are going to go ahead and just use here.*1737

*We said that the energy is equal to Planck's constant times that; well, we also know, from Einstein's relationship, that energy equals mass, times the speed of light squared.*1754

*Let's go ahead and put ν=c/λ; so now, we are going to sort of combine all of these.*1771

*Well, energy--so we have mc ^{2}=hν; so m, which is mass, equals hν/c^{2}; well, ν equals c over λ, so this is equal to hc/λc^{2}; the c's cancel; h/λc.*1777

*OK, now, wait a minute: what is going on here?--this refers to energy; energy refers to waves--energy is about waves; mass has to do with particles.*1812

*Particles have mass; waves have energy; this is suggesting that (these are constants here) light of a certain wavelength--of a certain frequency--actually has a mass.*1822

*As it turns out, it does.*1843

*We call this particle, if you will, of light--this particle--we call it a photon.*1849

*It is light, which we know is a wave; and yet, it actually behaves like a particle in certain circumstances; so the energy of the photon of light is equal to hν.*1861

*This is not just the energy, but it is the actual energy that is associated with this particle, when I see the particle as some energy, or I have energy that I can actually convert to a mass, based on this.*1876

*Light--electromagnetic radiation--has a particle-like quality associated with it, and you can actually assign it a given mass.*1890

*That mass comes from this relationship.*1901

*OK, so now, let's see if we can take it a little bit further.*1905

*We have: a wave can behave like a particle.*1913

*Well, it begs (well, that is not a question; we know that) the question "Can a particle behave like a wave?"*1925

*OK, can a particle behave like a wave?*1933

*In other words, can a particle have a wavelength?--can it have a frequency associated with it?*1943

*Well, guess what: the answer is yes, it can.*1948

*OK, so let's mess around a little bit more here: so we have this relationship, m=h/λc; well, what is c?--c is just a velocity (right?--it's a speed; we call it c, but it's just a velocity; it is meters per second).*1951

*So, there is a real-world analog to this for things that are not travelling at the speed of light; it's exactly what you think it is--just replace this speed with a velocity.*1968

*Mass equals Planck's constant, over wavelength, times velocity (I'll just put vel).*1982

*Well, if I rearrange this, guess what happens: I end up with λ=Planck's constant, over the mass times the velocity.*1990

*Well, mass times velocity is momentum, for those of you who have taken physics; if not, don't worry about it; you can just do mass times velocity.*2002

*Now, stop and think about what this is saying: this is saying that, if that I have a certain mass something, like a truck, moving at a certain velocity; if I take the mass of the truck, multiply it by its velocity (the speed at which it is moving), and if I take Planck's constant and divide by that, I actually have a wavelength associated with the fact that it is moving.*2009

*So, it actually can display wave-like characteristics.*2030

*Now, for large objects (like a baseball, a bowling ball, a truck, an asteroid...), the mass of these objects is so massive that the wavelength is virtually buried; we don't actually notice the wavelength.*2035

*But, as it turns out, as the mass gets smaller and smaller and smaller, when you get down to the level of neutrons, protons, and particularly electrons and smaller particles, the wave-like properties actually start to manifest in really, really strange and beautiful and clear ways.*2055

*In fact, the whole idea of electron microscopy is based on the fact that the electron is not just a particle, but it actually behaves like a wave.*2072

*If it didn't, we wouldn't be able to use the electron to actually see the things that we see.*2080

*That is the whole idea; so let's go ahead and do an example.*2085

*This is Example 3: We want to compare the wavelength of an electron travelling at 5.0x10 ^{7} meters per second, and a 4.50 gram bullet going 950 meters per second.*2093

*OK, so we are going to use this equation, which is called the de Broglie equation (after a Frenchman named Louis de Broglie): it actually gives us the wavelength associated with a certain mass moving at a certain velocity.*2131

*We have an electron that is travelling at 5.0x10 ^{7} meters per second (very, very fast), and a bullet, which is 4.5 grams, and it is going 950 meters per second (also very, very fast).*2151

*We want to see what the wavelength is associated with that; what is the wave-like property of that particle?*2164

*OK, so let's go ahead and calculate the wavelength of the electron: so it is going to be Planck's constant, 6.626x10 ^{-34} Joule-seconds...oh, actually, we can't use the Joule here; we have to remember...*2169

*Here is why: we have to make sure that our units actually work out; so this is going to be lambda; our unit has to be in meters.*2188

*So, if we recall, the Joule is a derived unit: the Joule is equivalent to kilograms-meter squared per second squared.*2196

*So, a Joule-second is equal to kilogram-meter squared per second (right?--Joule-second--the second on top cancels one of the seconds down below).*2208

*That is the unit; so it is going to be kilogram-meter squared per second; now, that means that mass has to be expressed in kilograms, because kilograms (the units) have to match.*2227

*The mass of an electron (when we look it up in a table) is equal to 9.1x10 ^{-31} kilograms (it's actually in the back of your book).*2241

*The speed is 5.0x10 ^{7} meters per second.*2253

*Here we go: kilogram cancels kilogram; meter cancels 1 meter; second cancels second; sure enough, we are left with a unit of meter; so everything works out just fine.*2258

*Well, the wavelength of the electron happens to be 1.45x10 ^{-11} meters.*2268

*And, if you want to take a look, that is on the order of the length of the wavelength of an x-ray--which is actually going to be important in just a minute.*2282

*So now, let's go ahead and do the wavelength of the bullet: so the wavelength of our bullet is equal to 6.626x10 ^{-34} kilogram-meter squared per second, divided by the mass (which, again, has to be in kilograms, so...) 4.5 grams is 0.0045 kilograms; and we said it's travelling at 950 meters per second--very, very fast.*2293

*Second cancels second; meter cancels one of the meters; kilogram goes with kilogram; and what you end up with is 1.5x10 ^{-34} meters.*2331

*There you go: you have a bullet that is a standard bullet, 4.5 grams; it is travelling at 950 meters per second; there is actually a wave associated with the energy that...*2345

*I'm sorry, it's not a measure of the energy of the bullet; that is different; that is kinetic energy (one-half the mass times velocity squared); this is different.*2358

*This is--the bullet--instead of thinking about it as a particle travelling through space, we are thinking about it as a wave travelling through space.*2369

*Now, that wave does damage; it does damage as a particle; but there is a wavelength associated with it.*2381

*The reason that we don't actually experience that wavelength, or some of the properties associated with this wavelength, is because the mass of the bullet is so big, and this is so immeasurably small, that it goes virtually unnoticed.*2387

*However, it does prove that (it goes to show that) a particle does have wave-like behavior, just like a wave (which is pure energy) has particle-like behavior.*2402

*This is the great--I don't know what to call it--the great unique thing about quantum mechanics--that, as you get smaller and smaller and deal with smaller particles at the atomic and molecular level, are you talking about a wave?--are you talking about a particle?*2415

*What is going on?--are you talking about pure energy, or are you talking about actual mass?*2433

*Well, both: they are just different sides of the same coin.*2437

*So, let's make some observations, and we will go ahead and close off this discussion.*2442

*1.45x10 ^{-11} is on the order of the wavelength of an x-ray--is on the order of the ϣ of x-rays.*2448

*As it turns out, shooting a beam of electrons...*2468

*Now, an x-ray is a wave; it is light; it behaves as light, and light has certain waves behave in certain ways that particles don't.*2477

*All of classical physics is based on the fact that waves behave one way; particles behave another way.*2491

*So, now, shooting a beam of electrons at a crystal gives a similar pattern to the diffraction pattern seen when x-rays strike the crystal.*2499

*When you shine x-rays at a crystal, the spaces in between the crystals actually take the light (which is in the form of x-rays; you know what light is--our general term--we are not talking about visible light), and it actually diffracts it, and it gives you a particular pattern.*2544

*Well, that pattern is unique and tells you what the structure of the crystal is.*2562

*That is how we get molecular structures for all of the biological molecules that we actually know what they look like--we actually crystallize them; we shine x-rays at them.*2567

*Well, here is what is interesting: this diffraction pattern is characteristic of waves--not characteristic of particles; particles don't do this; waves do this.*2575

*Diffraction is a wave property, not a particle property.*2584

*Well, check this out: when I take a beam of electrons (which are particles--we know that they are particles), and we shine them at the crystal, guess what happens.*2587

*We get the same diffraction pattern; that is very, very odd--that confirms the fact that particles behave like waves.*2595

*So, a particle (which, in this case, is the electron) behaves like a wave.*2605

*OK, so this was our general discussion: I'm going to go ahead and stop the discussion here.*2621

*The next time, we will go ahead and pick it up with a discussion of electrons in atoms, with an introduction to real quantum mechanics, and talk about electrons and how they behave.*2626

*But, I definitely wanted to get a good foundation, here, for light, how it behaves, the idea of wavelength, the idea frequency, and the idea of energy being quantized.*2639

*The big word is "quantum," and that just means packet--the Latin word for a little package.*2651

*The idea is that energy is quantized.*2660

*Energy rises and falls in discrete units; it does not rise or fall continuously, like some functions that we are accustomed to seeing.*2663

*It is very, very discrete; it is a stair-step function, is what it is.*2675

*With that, thank you for joining us here at Educator.com.*2679

*We'll see you next time; goodbye.*2682

2 answers

Last reply by: pak yin chin

Thu Feb 11, 2016 10:18 PM

Post by pak yin chin on January 20, 2016

Hi professor Hovasapian

I have question in my mid term exam cannot solve,

When sodium metal is bombard with ultraviolet radiation of wavelength 400nm,electrons with kinetic energy of 30.0 KJ/mod are ejected.What is the binding energy of sodium metal in KJ/mol and what is the wavelength of the enjected electrons?

thx

3 answers

Last reply by: Professor Hovasapian

Tue Jan 12, 2016 11:54 PM

Post by Tammy T on January 10, 2016

Hello professor Hovasapian,

-In ex 3, you said instead of thinking about the bullet as a particle travelling through space, we are thinking about the bullet as a wave (?=1.5x10-34 m) travelling through space. This was the part you mention something about not to confuse the Energy of the bullet as kinetic energy. -So I was playing around with the equation; I calculated Kinetic Energy of the bullet= 0.5mv^2= 2030.6J and the KE of electron to be 1.4x10^-15J.

-Then I calculate Energy from the wavelength of the bullet= h(velocity of bullet)/(wavelength of bullet)= 4192J. &Energy from wavelength of electron to be 2.28x10^-15J.

I dont know if it is correct calculate Energy from the wavelength like that. Supposed it is correct, what is the difference between KE and E calculated from wavelength? The numbers I shows that E calculated from wavelength is about 2 times KE.

2 answers

Last reply by: Jason Smith

Wed Nov 4, 2015 10:09 PM

Post by Jason Smith on November 1, 2015

Hi professor, this might be a silly question, but with science, you never know.

I understand that higher temperatures are associated with higher energy, and thus, higher frequency EM waves.

Does this mean that extremely cold objects emit radio waves? Since they're at the "lower energy" end of the EM spectrum? Likewise, does this mean that extremely hot objects emit gamma rays? Since they're at the "higher energy" end of the EM spectrum?

Thank you in advance professor.

1 answer

Last reply by: Professor Hovasapian

Wed Oct 28, 2015 12:36 AM

Post by Jason Smith on October 27, 2015

Hello professor, I have a question: is the energy associated with electromagnetic radiation the same energy associated with heat & work (from thermodynamics). I don't want to get too ahead of myself, I'm just curious :) Thanks!

1 answer

Last reply by: Professor Hovasapian

Tue Nov 25, 2014 2:27 AM

Post by Datevig Daghlian on November 24, 2014

Dear Professor Hovasapian,

As always, thank you very much for your lecture! My instructor told us that the speed of light is 3.00 X 10^8 m/s. Will this minute difference alter the results to the calculations I make on the AP Exam? Thank you and God bless!

George Daghlian

2 answers

Last reply by: Professor Hovasapian

Thu Nov 21, 2013 12:56 AM

Post by Tim Zhang on November 20, 2013

If molybdenum is irradiated with light of wavelength of 120 nm,

what is the maximum possible kinetic energy of the emitted

electrons?