- vibration. - vibration. and what's a wiggle in time through space called? begin with a w. - wave. - a wave. so we're gonna talk about wiggles in time and wiggles in space, vibrations and waves. we talk about vibrations, something may be bobbing up and down, or a pendulum swinging back and forth, or something that occurs and recurs. i could take my chalk on the board-- i can vibrate it up and down. i'm vibrating a little bit, too, okay? these are vibration, all right? now, how frequently i go to and fro has to do with frequency. we talk about frequency with the letter f. here's a low frequency. wanna see a high frequency? this is common-sense stuff. see that? ain't that neat? guess what a very high frequency is. how many say, "gee, i don't know what--" come on, you guys know. what's a very high frequency? [makes sounds] like that, yeah? okay. but when then a high frequency is propagated through space, watch. it traces out what we call a... sine curve. it's really a sine curve but a representation of a wave, okay? now, we gonna go through some definitions, gang. this frequency dragged out. the dotted line is my equilibrium point. i'm gonna vibrate above the equilibrium and below, and i go just as far above and just as far as below. i call this particular distance here, by the way-- let's talk about-- let's label these things. what do you call the maximum displacement of a wave? begin with a "a." amplitude. amplitude. you didn't even need the rest part. the other class, i have to say, "begins with the 'a,' ends with a tude." you know, they really go bonkers. okay, put tude. ah, then they get it, okay? we say it's amplitude. so we just put amplitude, "a." a lot of people think amplitude is from here to here. no, no, no, no. that's the peak-to-peak distance, the peak of the wave, the other peak. amplitude is the maximum displacement from the equilibrium position, yeah? - right. - these are just definitions. these are definitions so we can talk about sound meaningfully, and also, so we can talk about light meaningfully, 'cause they are both forms of waves, yeah? okay. we call this distance from here to here a wavelength. wavelength, l. now, we could put a l here, but does that look kind of exotic? l? doesn't it look sort of bland? wouldn't it be nice to jazz it up a little bit and use maybe a greek letter l? yeah? we can do a greek--anyone know what a greek letter l is called? no, nobody knows here. any greek scholars? - alpha? - no, not alpha. - lambda. - lambda. ah, lambda, lambda. that's right. and--watch, that's not so exotic, is it? your parents see that you're doing like this. they don't say, "l. i know the alphabet." but what if your parents look down and they see you doing this? "ooh, heavy, honey, heavy. you be learning that stuff?" you say, "i know that stuff," okay? that's just the greek letter l, it stands for wavelength. you see it all through wave nomenclature. so, you ought to know what it means. that's just a greek letter l, length, the length of a wave. so the length of the wave from peak to peak, or it's from here to here. this would be lambda as well. some people get mixed up. and they say, "oh, it's from here to here." but no. this part of the wave is going down, this going up. you gotta wait till it gets to a complete cycle. boom, down again. see? lambda is the wavelength that a wave will travel in a complete cycle, a to and to again. to, fro, to. see what i'm saying? so that's you're wavelength. and that wavelength, of cose, could be-- you can see it anywhere in there, usually peak to peak or from-- oftentimes, from here to here. so, this--the wave startrts, and here's where it ends. so this is a wavelength, wavelength, wavelength. kinda easy stuff. so when you talk about the wavelength of a wave-- when i say wavelength of a wave, we'll all be talking about the same thing, yeah? okay. there's another thing with a vibration too. there's something vibrating here. okay? now, the vibration has a frequency. what do you suppose the frequency is, gang? you say, "what do you mean what the frequency is?" it's a vibration, what you said." what's the value? what's the numerical value of the frequency? well, the frequency would be how many cycles per second. let's start here. about one per second. what do you get, lee? it will hit about one per second. one point what? one per second. well, let's just say, one per second. yeah. if it swings back and forth, one time per second, then its vibration frequency is one vibration per second. ain't that neat? or we can say one cycle per second. got that? or we could say, one avis-- no, no, no, no, wait, one hertz. [laughter] okay? one hertz is a shortcut name for one vibration per second. you get it? now, let's suppose i swing it back and forth two times per second. guess how many hertz i'm vibrating now. two. begin with a t. - two. - two. too easy? two. it's easy, huh? try this one. try this one. [laughter] let's suppose i vibrated back and forth seven times a second. hey. yeah, all right. 7.9 times per second. how many hertz? eight. [laughter] not eight. no, no, no. almost eight, almost eight. let's suppose we get really nitpicky now. 7.9 vibrations per second, how many hertz is that? 7.9. 7.9 hertz. you see what i'm saying? okay? now let's get to another item. not with the frequency of vibration is, but let's talk about the time during which one cycle occurs. this one is one vib per second, one hertz. how long does it take in seconds to undergo one complete cycle? begin with a w. - one. - one. okay. let's suppose i swing it at twice the frequency. how long would it take for one cycle? not everybody be seeing what most people be seeing. check the neighbor and see if the neighbor knows what the time is for one complete cycle, when it shook at two hertz. what's the answer, gang? how many say a half? all right, all right. let's suppose i told you i shake it back and forth... [makes sounds] ...seven times per second. what's the frequency? - 1/7. - seven. be careful, be careful, be careful. - do you see what happened? - yeah. [laughter] you all expected me to ask what's the--what's the time. i'm gonna call that time begin with the p. period. period. and you all expected me to ask you that. be careful in school because i didn't ask you that. i asked you a different question. what's the frequency? seven. next friday, when we take our exam, you start to read--"oh, i know what the answer to this is," even before you have the question. be careful, the question might be different than what you think it's gonna be. so, again, if i shake this back and forth, seven times per second, what's the frequency? - seven. - seven hertz. now, what's the period? - 1/7. - 1/7. so it turns out there's a relationship between frequency and period. frequency f is proportional to one over the period. in fact, it happens to be exactly equal to one over the period, if the frequency is in seconds-- or in hertz and the time is in seconds. and you can just write their reciprocals. see? i just turn it upside down. in your homes, the electricity that powers your appliances at 60 hertz, that means the electricity is alternating back and forth, guess how many times every second. 60. 60 times. what's the period? what's the time it takes for one to-and-fro swing? 1/60. 1/60th of a second. all you do is reciprocate the frequency and, gang, you got the period. well, reciprocate the period-- --boom, you got the frequency. do you see that? okay. so, those are important ideas. another important idea when we talk about waves is wave speed, how fast the waves travel and how fast anything travels is simply the ratio of how far compared to how long. so, we're talking about wave speed. it's simply equal to-- is this true what i'm gonna say? no. [laughter] what's wrong? is that true? wave speed, distance over time? gang, any kind of speed is the ratio of how far you go compared to how long a time it takes, right? well, let's talk about this wave that we generated that was kinda walk and it runs blackboard here. coming out of blackboard, okay? the--what takes on the distance? divide by the time, i got it. let's take a distance of one wavelength. that's the distance to go from here to here or from here to here or from here to here. if i know the time that goes by during the time a wave goes one wavelength, then i can put that in here and i can have the wave speed. if i were interested in things like that, then we kind of are, aren't we? and what is the time it takes for a wave to go one-- whole cycle? what do we call that time? begins with the p. - period. - period. okay? and so all over here, we can say wavelength over the period of the wave will give you the speed of the wave. but the period of the wave can be re-written, one over the frequency. and those of you who are a little bit into mathematics know that this is equivalent to-- all right, let me write it this way. and so we have a relationship for the speed of any kind of waves, gang. and it simply is the velocity of the wave, the velocity over the speed, speed is the magnitude of velocity. that velocity of the wave is simply gonna be equal to the frequency of the wave multiplied by its wavelength. later on, we'll talk about the speed of light. and the speed of light will be equal to the frequency of light multiplied by its wavelength. so if you know the wavelength of light, you can calculate the frequency, if you know the speed. or it turns out light speed will always be the same. do you know how fast light goes, gang? it goes just as fast as a radio wave goes. it goes just as fast as an infra red wave goes, as ultra violet wave, gamma ray, all the electromagnetic waves. guess what we'll gonna be talking about later in the course, gang? electromagnetic waves, they all have the same speed, that's 300,000 kilometers per second. in fact, for light, we say it's a constant, so we say, c. c for constant, that's 300,000 kilometers per second. and that's equal to the frequency of the wave multiplied by its wavelength. questions? what is the label for the--for-- to... say, again? the label for each, it has to be a certain thing, right? like for your distance, shouldn't it not be-- on labeled in meters or something? oh, yeah. if this is in meters, and this is in seconds then i get meters per second. and i'll have my speed in meters per second. it turns out, for a light wave, you go a hundred--you go 300,000 kilometers in one second. so you'll have distance and meter or kilometers or micrometers, or miles, yeah, and t and some unit of time. could be like days, huh, or could be hours talk about miles per hour, meters per second, all those thing. but the unit of distance, a unit of time. so will the formula work for any unit? you might have to fudge around your units. let's suppose your frequency is in cycles per second, okay? and you want the speed in miles per hour, then you've got to change seconds to hours and you do that kind of thing in a lot of courses. you have to convert from one to another. but if you have this for example in cycles per second and this in meters, then the speed will be in meters per second. if this is cycles per second or hertz and this is in like, millimeters, then your speed will be in millimeters per second. and it kinda makes sense. but this is the relationship that these are tied together like that which is kinda nice, you know? in general, we just say speed and c was for the speed of light. on the 300,000 kilometers per seconds, is that a convention for the convenience of math or that the actual speed. it's almost that actual speed. it's 299 and i can't remember the numbers, but it's very, very close to 300,000 kilometers per second. yeah, it's not--now, that's rounded off. and you will find that later on in the book, what the presently accepted value is. it's close to that. did you ever wonder about a radio wave, how long maybe the wavelengths might be? the wavelength we saw over here is about less than a meter. there are radio waves in this room, gang, you'd be knowing that. how long are these waves? are they tiny, tiny? are they big, big? well, we can find out. we can find out because if speed is frequency times wavelength and the speed of a radio wave-- does anyone here know what the speed of a radio signal is that come from the radio antenna's downtown? anyone here know what the speed of those signals are? i'll bet there's people is from that know. in fact, i'll bet that most of the people in this room know, and they say, "wait a minute. isn't that the speed of light? did he said the speed of light?" how many people saying, "yeah, i think it's the speed of light." "no, a speed of light is for light. this is a radio, honey. it's different." no, no, no it's not different. it's just the low frequency. we're gonna learn later on just the low frequency light wave. and it all have the same speed. so the speed 300,000 kilometers per second equals to frequency of the wave times the wavelength. give me your favorite radio station. what's the call letters? kccn. kccnn. what's the number for kccn? - 1420. - what is it? - 1420. - am. what, 1420? am. is it 1420? let's see how long those waves there. the frequency, gang, is guess what? 1420. can we round it off to 1400? - i can't do, too, huh? - it's not. it's gonna-- oh, that's 1,400 kilocycles, right? oh, kilocycles. kilocycles, that's another thousand, that's the frequency. and now we're gonna get the wavelength. well, we got this multiplied by this will give this. do you know there are people who know how to find this knowing the other two? [laughter] there be. there be. they're called engineers in science types, in physics types. in fact, they're us types, aren't they? what's the wavelength, gang? let's go. watch this, one, one, two, two, three, three, four, four, five, five, now it's 3/14. okay. so wavelength equals 3/14 of a what? - kilometer. - kilometer. okay? that's like 3,000 meters divided by 14. why did you pick a four, man? i mean--how many times does 14 go into 3,000? does anyone have a calculator? 214.28571. 214 meters long. is that surprising to you? that's like two football fields and then some. so the wavelength of your favorite radio station is more than two football fields long. radio waves are long or short compared to light waves, gang? we'll do the same thing for light waves later on and find out it's a smidge, smidge, smidge, smidge, smidge. zero's go the other way. yey. anyway, that's how you find the wavelength of something. i can show you the slinky here. can someone grab the end of this for an "a" in the course. [laughter] right over here. now, i'm gonna take this wave and i'm gonna shake it back and forth along the direction like this. well, i'm gonna expect a little bit more. if i vibrate it like this, can you guys see that the vibrations are along the direction of travel? we have a name for that kind of wave. first, there's two kinds of wave we're gonna distinguish, longitudinal and transverse. what kind of wave you suppose it is that vibrates along? longitudinal-- along the direction of wave travel. how many say, "oh, it's probably a trick. "it'll be transverse 'cause it seems like common sense say longitudinal, right?" what's the answer, gang? - longitudinal. - longitudinal. how about if i vibrate it transverse to the direction of wave travel? it's like the kind of wave i have here. what kind of wave is that, gang? - transverse. - transverse wave. and both those ways have a wavelength, frequency, period, wave speed and the whole shebang, all the characteristics of waves. thank you, david. ted's got some nice things here to play with, gang. ted bradstraw, our ta. okay. waves are all part of the game and--we're not loud enough. here we go. do we see waves or do we see waves? yes. okay. we can also have voice waves and they look much more interesting. they go-- [makes sounds] [laughter] and you can get complicated wave forms and nice easy wave forms. you can make whistles. [whistling] that's good. let's go. you know that everyone has their own pattern like your own fingerprints. hold on. where are you going? go way on me. hello. testing, hello, hello. i think that's good. hello, hello. ha, ho. lots of fun. i admit-- i'm a great singer. maybe we could show the idea of interference. okay. i wanna do here is-- first of all, i have to show the students this happens with-- do you know--do you guys know that one wave can interfere with another in an interesting way sometimes? let me show you-- for water waves. right, the water waves. let's suppose i take another water wave and add it right on top and the other wave is exactly the same frequency, same wavelength, save everything, okay? but watch this. when those combine together, guess what i get? is that remarkable? that's not so remarkable. you kind of expected that. but you want me to show you something that is remarkable? you'd like to see it? --let's suppose i add another wave but i do it-- i start out of step. by the way, physics types never say out of step when they talk about waves. they use a little more pizzazz. out of--begin with an f, out of what? - phase. - out of phase. and if those two waves are completely out of phase, guess what the combination produces? nothing. nothing. that means if i take these two tuning forks which are tuned for the same frequency-- i know that because it just resonated-- and i strike one and then i strike the other just out of phase, guess what you're gonna hear? nothing. guess. begin with n. - none. - nothing. would you like to hear one sound destroy the other? - yeah. - i can do this. listen to this-- i'm usually good at this. it requires that i hit this one, and as it's going, with my judgment, hit this one right. when this one is sending out a high pulse, this one will send out like rarefaction. did you read about the difference between compressions and rarefactions? you know, when i strike this thing, the waves--boom, boom-- compressed air comes out. the compressed air, compressed air, compressed air, followed by decompressed air? watch this. now, listen. [laughter] hear that? did you hear the absence of sound? - did you hear--was that neat? - - no. i never do it more than once because i don't wanna trust my luck, but you get the idea. but i'll tell you want. let me get these things so they're a little bit different in frequency and strike them both at the same time. can you hear the throbbing? - that is a result of inter-- - ference. --interference and the phenomenon that's called-- sound. this phenomenon of pulsating-- two sounds pulsating in and out, in and out of phase is called, by the way--scholars? - beats. - beats. all right. i thought you guys were putting me on there for a while. and that's a beat. ted, can we show beats up here? hopefully show beats. so you can see as well as hear what's going on. you can see the wave form. isn't that nice? we certainly hear the wave forms but now we can see them. so this device, called an oscilloscope, beautiful, allows us to see the shape of waves. very, very nice. wow. where it's extra loud, a compression from one fork and a compression from the other have reached your ear. and where it's very low, a compression is matched by a rarefaction, in one fills and the other, it makes it like null. isn't that nice? beats. question, paul? can you explain why laser discs are suppose to have a better sound quality than tapes? those laser discs amaze me. let me tell you what a laser disc does. you saw ted sang in here and showed you a particular wave. well, when ted did that-- ted maybe made a wave like that. now, what i wanna do is i wanna record that wave and give it back to you. the old way of doing it was just to have ted speak into a microphone, and when he spoke into the microphone, a little crystal would start to vibrate too. and the crystal vibrates like that, the electrons get squeezed up here and spread out here so that just pass through the wire. and as the crystal vibrates, the little electrons will vibrate too, just in rhythm, by the way, 'cause the electrons have hardly any inertia. and so you get an electrical signal and that would be brought over to a little needle that'd be riding on a waxed disc. and the needle would vibrate the same way and those vibrations would be caught up in the waxed disc. and you play it back and you-- hoop--you get the same tone. that's the old way of doing it. well, now they say, you want the signal just to go from here-- at this point here, you wanted to go that high, here this high, here this high, this high, this high, this high. why don't we just measure the distance from here to here and send that through? and that's what a laser disc does. when it gets to this point here, it just tells you what the amplitude of the wave is and it does that with little zeros and dots. and says--here to here. poom, a short time later, it's up to here and it sends you what the amplitude is there. zeros and dots, it's a number. then now it's up to here and to here and to here. so all your laser disc is doing is just counting up the amplitude as it goes along and tracing out the wave. it's amazing. that's what it does and there's nothing to wear out now. 'cause the old record would've finally get a little, you know, the little-- you know, get a little worn out. but all the laser disc does is it tells you how high the wave is at different points, period. and then what-- then you play it back-- another interesting difference between a laser disc and a regular disc, the record player. by the way, these records-- some of you guys still have some records at home, right? hang on to them. they are collector's items. you're gonna show them to your kids and your kids are gonna say, "wow. were you around when they had those things?" [laughter] you say, "i used to dance to those things, honey, okay?" yeah, they're gonna go out. hang on, you won't see them very much longer. but a laser disc doesn't travel at the same speed. i didn't know that until recently. you know, a regular record travels at the same rpms, yeah? so out in the end, it's going faster. isn't that true? but on a laser disc, it travels at the same linear speed but different rpms. when you're in at the center, that's where it begins, by the way. a laser disc begins at the center and works out. another upside down, yeah? and when it's at the center, that thing is spinning faster. and when it gets out here, it's spinning slower. so that the message goes under the laser beam at the same speed no matter where you are. you don't get enough of that on a phonograph if it has a constant rpm. on the edge of the phonograph, that thing is racing under the needle. that's where your best fidelity is out there. and as you get closer in, now that part there, that is going slower. isn't that true? know guys know what-- the inside of the wheel goes like slow and the outside will keep up going faster. laser disc is just the opposite. laser disc are amazing. so many things-- so when you get your laser discs, you say, "well, this is as far as they're gonna go." and they'll never have anything more than this, right? and down the road, they'll say, "oh, laser discs, honey, "you remember those old fashioned things? now we got--" and what's it gonna be, gang? what's it gonna be? i'll tell you one thing that still seems to persist. i've been waiting for years for it to change. and when i was a little kid, i remember i use to look at cars and i look at a car and i say, "whoa, that car there, they can't design a more beautiful car than that." the designer types are at the top. that model t, by gollie. no, not quite, okay? but there's one thing about the car i always wanted to see change. that's the windshield wiper. how do you get the rain off of your windshield? [makes sounds] there's got to be a better way than that. and i've been waiting and waiting and waiting to see what is gonna get the rain off the window. and when you see these great, big 747s come in, or you see the space shuttle come in, do you know how the space shuttle gets its rain off the windows, gang? [makes sounds] [laughter] they still have the same kind of windshield wipers. isn't that something? someday, someone's gonna come up with something that's gonna makes the windshield wiper obsolete. and they will say, "oh, remember, "they used to get the rain off the window "with the windshield wipers? they don't do that now. now they have a--" the one who finds out gonna be rich. what is it, gang? here's your challenge. class project: think of a device that's better than a rubber windshield wiper, okay? i've been watching for years for it to come, hasn't come yet. i wanted to talk about, like the doppler effect and the sonic boom and shockwaves and--waves. but we don't have enough time now. we'll do that next time. okay, gang? catch you then. [music] captioning performed by aegis rapidtext the big game in the world is the movies. it's the biggest game. it always has been the biggest game. television is the exact opposite. it's a postage stamp and it has to draw you in. there's no question that this is the age of images and it became that way because of television. and the movies, of course, have to deal with that. i think we're on the verge of a media revolution comparable to the arrival of television itself. annenberg media ♪ and: with additional funding from these foundations and individuals: and by:

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