 One thing I wanted to show you at the beginning here to sort of tie up this business on on magnetism and the biology of magnetism was to show you a video that I mentioned a couple of lectures ago about how you can knock out the speech center of the brain by exposing it to strong electromagnetic radiation. Okay, so I'm going to cut the lights and show you the first part of this video. And this really I think illustrates the fact that understanding electricity and magnetism and understanding the role that plays in biology is important because you may need to use it as a tool to treat something in the future or to study a disease of the mind in the future at this disability of the mind, a change in the way the brain behaves, whatever. You may have to employ techniques involving electricity and magnetism. So here we go. That may have looked like a strange party trick, but it's actually part of the serious research project conducted by Prof Vince Walsh in a basement lab at the Institute of Cognitive Neuroscience in London. He's using electrical and magnetic brain stimulation to understand the most complex known object in the universe. Big coil of wire being put against the head. You know what that thing is generating. Just to show you the power of the magnetic fields passing through my scalp and that this wasn't a trick, this is what it did to our video It's hard to see because of the interference, but here I could still sing a nursery rhyme even when he disabled my language centre because singing depends on the other side of the brain. I felt a bit apprehensive during this intriguing demonstration that Prof Vince Walsh makes nothing of tinkering with his own brain for the sake of science. That's a scientist right there. If people are interested, I'm pretty sure I pulled this off of YouTube, the new scientist, to put this up as a video so you can watch the rest of it and see what he's actually trying to do with his research. That gives you a sense of how tenuous and frail our existence is that you could knock out your speech centre just by putting a magnetic field right over here. But singing is controlled by a different part of the brain that's further away from the interference and so it doesn't get completely knocked out when you expose the brain on that side to a strong magnetic field. It looks like tinkering but presumably there are things we'll learn about the functioning of the brain in the way that information is distributed and the responsibility is distributed through the brain by doing functional MRI imaging by doing experiments like this, I guess on one self. Yeah, Raza. Why does the magnet kind of like wipe the brain like the computer, you know, you can wipe a computer with a magnet? Well, because in a computer, typically one bit of information is stored as the orientation of the magnetic dipole of a single location on a disk. The whole magnetic disk would have used a magnetic field to flip the direction of a magnetic material and, you know, one direction would be one and the other direction would be zero and you could encode data by ones and zeroes. Knock out any one or two of those bits and you corrupt the data. In the human mind, information is distributed across the brain. You don't remember events. You piece the things you do remember back together and call that a memory. All right. So this is why, for instance, eyewitness testimony is such a frail thing because there's a great TV program on eyewitness testimony and its fallibility. And they demonstrate that people not only influence each other by what they say, though people will convince other people inadvertently of things they actually didn't see. That also turned out to be false. Okay. But also people will piece together facts at different times in different ways and come up with different recollections of the same event, the same person. So you have a distributed information system with redundancy built into it. A hard disk, you're trying to cram as much data as you can into one place. Redundancy exists, but it's not like a biological system. So if all you have to do is flip the region of magnetic field and that's it. That data is lost forever. You can never recover its original orientation. Solid state drives do store things a little bit differently. So this one, for instance, has a solid state disk in it, which uses essentially transistors instead of magnetic regions to store data. Flash drives, the modern iPods, the modern iPads, they all use internal solid state drives. They don't use magnetic drives anymore. They're relatively impervious to the kinds of magnetic fields that used to disrupt these things. You can destroy your credit card information by putting in your wrong magnet. That doesn't have redundancy built into it. So the human brain is very different from magnetic, man-made magnetic storage devices, let's say. Okay, so today I'm going to try to illustrate how everything we've been doing for the last four-fifths of the class comes together and can give us an explanation of the nature of light. Light is a ubiquitous phenomenon. It is certainly one of the first things that we become acquainted with when we start to see color, shape, depth. Facial recognition through pattern recognition in our brains. Light is invaluable to most of us to making it through the course of a normal day. But really, to begin a discussion of light, one has to start asking questions like, what is light exactly? So you tell me, what do you think light is? Anyone want to, Ashley, you want to offer me a, what do you think light is? Yeah, it's kind of working back to what's electric charge, right? Okay, so you've been exposed to the idea that it's waves and that those waves, so what do the waves do? Travel. Yeah, it's some kind of phenomenon and we'll get to the wave nature in a bit and how exactly that came to be understood. But now the modern understanding is that, yes, light is some kind of wave and that most fundamentally it's a phenomenon which departs some other location, arrives at our eyes and our brain has mechanisms in it that have evolved over thousands of years to convert that light into information that tells us about depth, distribution, color. Our eyes, however, are very narrowly sensitive. There's so much more light out there in the cosmos than our eyes can show us. And so a lot of the last 50 to 70 years of human technology have been to open up our eyes to wavelengths that we were never sensitive to before and in doing so reveal the universe we didn't even know existed because you couldn't see it, okay? So, yeah, most basically light is a phenomenon. We now understand it to be a wave phenomenon, although I'll come back to that a little bit at the end. But most basically it is something that travels from a source to us and we can interpret it in the same way that sound is something, right, that travels from my hands through a medium which we now understand it's basically its displacement of air molecules in a wave, a pressure wave. The wave travels across the room, strikes Lashley's ear drums which then vibrate in response to the pressure wave and the mechanisms in her ear and the electrical connections from the ear to the brain then convert that into something your brain recognizes as sound. So sound is also a phenomenon that travels from one location to another. Does sound travel through empty space? If I take all the air out of the room so there's no way to create a displacement of air molecules can I still make a sound? Yeah, no. All right, so if you're on the moon and you clap your hands there's really not a whole lot of stuff for a pressure wave to travel through on the moon so you wouldn't hear anything. I wouldn't expose my ears to the vacuum. That would be bad for other reasons. Your blood would boil. But if you could do that you wouldn't hear anything. Water waves. Water waves are another excellent example of a phenomenon that starts at a source. There's a displacement in a medium. It's a pressure wave again in the water molecules and that transmits energy in some cases as we saw with the Japanese tsunami over vast distances and can carry all that energy back into the form of mechanical energy. So it can build things. It can pull people out to sea. These things are very destructive. Loud sound waves are also very destructive. You know you can blow out your ear drums by being exposed to extremely loud sounds either over a long period of time or so loud that they just shatter your ear drums. So waves are good and waves are bad and like anything else you want them in the right sweet spot to not die. Let's talk a little bit about light and some of the characters that were involved in the history of light. So I would argue that up until this person, Galileo Galilei the understanding of light that people had was pretty much a bunch of myths and misconception and made up stuff because no one had bothered to try to experiment on it before. And it was, in fact, this idea that you can do an experiment and reveal the character of the natural world really first was articulated by Galileo Galilei. So he is considered to be the very first modern scientist, somebody who articulated a version of the scientific method as a means to make statements and predictions about the natural world. So he was a brilliant mathematician and he combined his understanding of mathematics with keen observation and argued that this was the way to understand the natural world. So mathematics, prediction, observation, measurement and combined the two to come to some narrative about the world that can be consistently arrived at by anybody else that applies the same techniques. Now his big claim to fame early in his life was that he perfected the telescope. So like any good scientist, he took something that already existed. The telescope was not his invention but he improved on the lens making process of the telescopes and made them into a military weapon. So like any good scientist, he weaponized it and sold it to the government. And he made a ton of money and got a lot of fame off of it. Basically he gave the Italian government the ability to spot ships coming in from the Mediterranean long before the ships knew that they had been spotted. So port cities could prepare for attack or send out a force to retaliate before they even reached the shore. Being able to see your enemies at a great distance when they can't see you is a strong military advantage and enable society. And so he got very famous and made a lot of money off of this and basically got to do whatever he wanted and so he was taken on as a court scientist by a very powerful family in Italy and they just gave him money and freedom. He had to teach and of course he had to serve the family's needs as a naturalist but basically he got to build his own experiments at that point. So what he did was he took his invention which essentially now had been weaponized and he said, not interesting, and he turned it to the sky. Now there were a lot of ideas about the sky and its perfection and about the nature of the earth and its role in the cosmos. It was a very popular belief that the earth was the center of the universe and all things went around the earth. Now prior to Galileo the notion that the sun is probably really more likely the center of the universe was posited but it was Galileo that really cemented observationally that the idea that everything goes around the earth was a false idea because he turned his telescope to Jupiter and he realized that there were tiny little, he called them wandering stars that seemed to orbit only Jupiter. They didn't go around the earth, they went around Jupiter and that broke immediately the idea that everything in the heavens goes around only the earth. So he published this book on these wandering stars and he became very famous as a result of these observations. He was the first to observe that the sun has spots. The sun was believed to be unblemished and perfect. Seeing spots on the surface flew in the face of biblical interpretation at the time. He got into a little trouble but other people confirmed his observation. Galileo had quite a mouth. He wouldn't shut up and he really believed that he was right and he got himself into a lot of trouble because he collected all of his observations into a great work and that work he published in the form of basically a conversation amongst three friends. One who mediated the discussion of the other two. One that mouthed his observations and the other named Simplicio that mouthed the positions of the Roman Catholic Church. The pope who was a friend of Galileo's wasn't too keen on the idea of the mouthpiece of the church being named Simplicio and the simpleton, which is what that means. And so he was dragged before the Inquisition, the Inquisition. He was condemned as a heretic and asked to repent or face death. So he got on his knees and he repented and he was put under house arrest for the rest of his life and he died in his home. But when he was under house arrest while he was ill he had a lot more free time again and he started exploring motion and space and time in ways that when he was younger he just hadn't bothered to spend the time exploring. And he came up with some interesting ideas about what space was and what time was as a result of that. He died a year before Isaac Newton, perhaps one of the greatest scientific minds in human history, was born, which I think is just very interesting. Now Galileo was very interested in light and he wanted to know if light was an instantaneous phenomenon. That is that from the time it's emitted to the time it's received is zero time. He wanted to know if light travels at a finite speed. So he proposed an experiment and that experiment is something that the physics of musical instruments class here at SMU does but they do it with sound. He proposed that he sit on a hill with a lantern that is covered and he has an assistant, probably the equivalent of a graduate student these days, go to a very distant hill that's still visible with another lantern that's also covered. Galileo had built very sensitive water clocks that he could precisely do timing with and he proposed that he uncover his lantern. Now the light is free to travel to the other hill when the assistant sees the light from his hill he uncovers his lantern and when Galileo sees the light from the other lantern make the return trip he times it. So he starts his clock when he uncovers his lantern he stops his timing when he sees the light from the other lantern. That distance is far too short on the surface of the earth to make a measurement of the speed of light to even say whether or not it is finite. The furthest you can see because of the curvature of the earth is roughly 60 miles depending on whether it's a clear day whether you're up high and things like that. Sound on the other hand travels about what 300 meters a second as opposed to light travels extraordinarily much faster. So you can actually go to the steps of Dallas Hall and Professor Fred Oleis has marked out the distance to the flagpole and to a point further down the boulevard here. And he knows that this is almost exactly 100 meters and then you go and go another 100 meters down this way. So you have a team of students here with an air horn and somebody in the first team with a stopwatch. So the first team fires their air horn and the stopwatch starts at the same time. When the team at the flag hears the first air horn they fire theirs in return and then the first team when they hear the return horn they stop timing. And you can do that measurement and then do it 200 meters away to take out human reaction time as an uncertainty for about 1-2 seconds. And you can get a very precise, maybe 5% or so uncertainty measurement of the speed of sound over just 100 meters away and 200 meters away. I want to do this in Fort Stadium someday. I think that would be a lot of fun. Don't get shot down by the cops. One of our very enthusiastic new graduate students last year was in charge of this lab and he had them firing the air horns a little bit too much and he was standing here with a little pink card which he would hold up to signal the first team should fire their air horns and I was standing here taking pictures of some students that had won some honors so we could put them shamelessly up on our web page and three police vehicles pulled up around that graduate student and circled them. And they told them that you shouldn't be making noise on a college campus in a post-911 world. Go figure. Okay? So, careful. Science can get you thrown in jail. So luckily we popped our way out of that one but now there's a big form with 22 signatures you have to get in order to do the experiment. Now, Old Romer. Old Romer in 1676. So I should say if we go back here to Galileo so he died in 1642. So just about 30 years later Old Romer was the first person to measure that the speed of light was at least finite. Didn't quite get it right and actually if you re-look at his data now he misinterpreted his data a little bit but you can definitely, no matter what from his data you can conclude that the speed of light is not infinite. It doesn't go instantaneously from source to receiver and he did this by being here on Earth and in fact in a sort of beautiful twist of science he looked at Io which had been discovered by Galileo as a moon of Jupiter in 1610 and he measured the orbital period of Io around Jupiter at different times of year for our orbital period around the Sun. And at different times of year we're at different distances from Io so light has to travel further or shorter in order to get to us so we can make the measurements and by looking at the variation in the measurements he was able to place a bound on the speed of light he bounded it at, he estimated it to be about 220,000 kilometers a second which actually isn't too far off the mark that's pretty good, alright? So what is actually the speed of light? Anyone know? 3 times 10 to the 8 meters per second 3.0 times 10 to the 8 meters per second I'm going to go one further and say it's like 2.998 times 10 to the 8 meters per second alright, no, not too bad for good ol' Romer here he did pretty good getting close to that okay, let's go back and talk about what we've been doing for this whole semester there are four fundamental laws of electricity and magnetism which are collectively known as the laws of electromagnetism and that name is not an accident it's not just for convenience we now consider these to be a single phenomenon Faraday's law which we've just learned that a changing magnetic flux induces an electric potential difference in a conductor now, I showed you an alternative form of this I substituted with electric field in for the potential and you can actually write Faraday's law in what's considered a much more fundamental form a changing magnetic flux induces an electric field and after all an electric potential is just an electric field over a displacement okay, so when you say a changing flux induces a voltage what you're really saying is it induces an electric field so changing magnetic flux can cause electric fields to occur there we go we also have Ampere's law and Ampere's law is really just the B.O. Savart law in another piece of clothing but it's a simpler, more compact form and it's traditionally the form in which this law is written Ampere's law tells us that electric current is the source of magnetic field moving electric charge I is the source of magnetic field B now, way back at the beginning of this class we have this thing called Coulomb's law which was similar to Ampere's law in that it says that electric charge is the source of electric field okay, so Q is the source of E now, I skipped this chapter but this law, much like the B.O. Savart law can really be couched in this thing called Ampere's law there's a more simple and fundamental equation than Coulomb's law and it is known as Gauss's law for electric fields and all Gauss's law says is that if I take a region of space and I enclose it in an imaginary three-dimensional surface like a sphere that if I take the dot product of the electric field penetrating a little piece of the area of that sphere and I sum up all of those dot products across the surface of the sphere if I find that that integral, that sum, is zero no charge is enclosed if I find that that integral is non-zero then there is electric charge enclosed this is just another way of saying electric charge is the source of electric field it just says the same thing in a different mathematical form and finally there's also a version of Gauss's law for magnetic fields Gauss's law for magnetic fields says that as far as we know there are no individual north and south poles there are no individual magnetic charges and therefore if I take and do the same integral for a magnetic field penetrating an enclosed three-dimensional surface I will always find it to be zero because there are no magnetic charges enclosed I can never enclose a bare magnetic charge so this integral will always be zero that's a really nice law that's a nice easy one we'll be under us if we ever find out there are magnetic monopoles because that's going to make this equation look like this basically but with some other constant in front of it these are the laws as they were essentially understood before a man named James Clerk Maxwell came along and tried to understand what these four laws were actually describing if you recall it was Michael Faraday that initiated this concept of the field as a force per unit charge that reaches out from one charge to another charge and communicates a change in energy or an acceleration a change in spatial position to the other charge and causes it to move so that Faraday's concept of the field is beautifully embedded in all four of these equations two of them for E two of them for B now B also appears up here we'll come back to that in a bit on the right hand side this is the one equation where one of these fields B is buried in the magnetic flux B dot A that's magnetic flux B dot A so B is buried in that right hand side now Maxwell among others considered the following question what if I were to take these equations that I've just written down let me see if I can go back ok here we go so what if I take these equations that I've just written down and I consider them in a region where there are no charges and there are no currents this is called free space so picture the empty vacuum of space with no matter anywhere presence in the region that you're looking at so you make a volume like an imaginary box in space and you say ok any atoms, no atoms any electrons, no electrons this is free space there's no matter in here whatsoever in that case Q is zero and I is zero and these equations beautifully simplify these are the four laws of electricity and magnetism in free space these are really nice right you've got B dot D A equals zero E dot D A equals zero integral B dot DS is zero yay Ampere's law is easy ah ok well let's see this is just changing flux flux is just magnetic field penetrating an area that doesn't require that there be charges or currents inside my volume so that flux may not be zero I have to leave that there ok and what Maxwell kind of recognized as he looked at this was something's amiss there's something wrong about these equations somebody who listens to the language of mathematics too long looks at these four equations and thinks well these make sense these are nice and symmetric in empty space E dot D A that integral is zero B dot D A that integral is zero that's nice but here this path integral E dot DS is something that may be non-zero and this path integral of B dot DS well it's always zero this sort of breaks a lovely symmetry in these equations so Maxwell who was this brilliant scientist working in Britain had a hypothesis he hypothesized that actually this equation is incomplete that there's a piece missing from the right hand side that nobody had observed before and that it is related to a changing electric flux so he hypothesized that a changing electric flux oh I almost did it induces a magnetic field in the same way that a changing magnetic flux induces an electric field he envisioned that symmetry may be required and he made a prediction he predicted that a changing magnetic a changing electric flux would induce a magnetic field so that was a prediction that he made based on looking at these equations and saying something doesn't feel quite right among his many accomplishments he lived from 1831 to 1879 well as you'll see in a moment he united electricity and magnetism into a single force he developed the theory of how large numbers of particles will behave he was very interested in thermodynamics and the study of large systems of gas particles and things like that he made the very first true color photograph this is him actually holding a color wheel and in 1864 he published what was perhaps the most important paper in the 1800s to set the stage for everything to follow and it was entitled a dynamical theory of the electromagnetic field and dynamical meaning time changing and that is what he essentially insisted had to be over here a time changing electric flux inducing a magnetic field that piece is missing because we haven't observed it let's predict that it exists and go see if it exists and so experiments were essentially done to do that so for instance you could take a capacitor and you could change the strength of the electric field inside a capacitor over time that changes the electric flux penetrating an area inside the capacitor and you could look for the creation of a magnetic field and in fact this was observed and so Faraday proposed that in fact after working through the math a little bit to see how one would relate this path integral to this time changing electric flux that these in fact were the four equations of electricity and magnetism in free space a time changing magnetic flux induces an electric field a time changing electric flux induces a magnetic field and in free space there is no source of these electric fields but you can still have time changing electric and magnetic field passing through that region of space that's what these two equations tell you okay so what you notice is that here you have these lovely constants again the permeability of free space mu knot and epsilon knot these are just numbers we've been using so far they were empirically determined by experiment but they're just things we've been writing down so epsilon knot is actually hang on, help me out here is it 8.99 times 10 to the no what's epsilon knot K is 8.99 times 10 to the 9 Newton meter squared for Coulomb squared 8.85 times 10 to the that's it, thank you 8.85 times 10 to the negative 12 and that's Coulomb squared over Newton meter squared okay and mu knot mu knot is a little bit easier to remember it's 4 times pi times 10 to the minus 7 and that's going to be Tesla meter squared per something something per amp, thank you okay very good alright so those are these numbers we've been throwing them around just as constants but it was through this work of Maxwell that we finally understood what these were so Maxwell said okay I have some differential equations remember I was teaching you a life skill last time you have a time changing magnetic field inducing an electric field a time changing electric field inducing a magnetic field these are a coupled set of differential equations what one induces in its left side becomes the right hand side of the next and induces its left side back into the right and induces more on the left and so forth so just to illustrate this if I cause something that creates a changing magnetic flux this induces an electric field if that change is happening at a different rate over time this electric field changes strength aha a changing electric field is a changing electric flux which induces a magnetic field if this rate of change is changing over time then this will change in strength which induces a change you get the idea these equations are coupled to one another in time what one does influences the other and what that one does influences the first Maxwell recognized this and he solved these equations what are the functions that solve these differential equations yeah that's the right you know whoa right looks like somebody put a magnet coil next to your head make your eyebrows shoot out relax here's what we have we have some magnitude some constant some maximum strength of the electric field some maximum strength of the magnetic field this is the electric field as a function of both space x and time t same here magnetic field is a function of both space x and time t what is the time relationship well he found that it was sinusoidal this is the sine function it acts on both space x and time this number here k is called the wave number it basically tells you the number of waves per unit meter so the number of crests in a sine wave per unit meter so that's a counting so if I have a wave and I have a meter I count 1, 2, 3, 4, 5 there are 5 crests per meter that's the wave number omega is the frequency it's the rate at which crests pass you as you're standing there ok so if I count 1, 2, 3, 4 in 4 seconds then I know that the frequency with which crests of the wave pass me is about a hertz or 1 per second that's the unit of frequency the hertz and you'll see why that is in a moment now what he found was that these solutions have direction the electric field points for instance in the y direction the magnetic field points in the k direction but the wave propagates along x so the wave is propagating in a direction perpendicular to both e and b so what do these equations describe? well they describe waves as I just said so this is a generic equation for a wave the wave number k is 2 pi over lambda lambda is the wavelength of this wave actually it's about from crest to crest about 2 meters probably on the screen a little bit over 2 meters ok so this is about a meter from here to here ok so a little over 2 maybe 2 and a half meters is the crest to crest distance on this wave f is the frequency and again that's in hertz so we'll start counting here so 1, 1,000, 2, 1,000 seconds alright so that's one wave going by every 2 seconds or half a hertz ok 1 over 2 half a hertz ok so this just illustrates what I was saying these are the crests of the wave these are the troughs of the wave the distance between peaks and the crests is the wavelength and the frequency that crests past you is this frequency f these are electromagnetic waves with an electric field in one direction a magnetic field orthogonal to that and the direction of travel perpendicular to both so as the electric field grows in strength it induces a magnetic field which also grows in strength as the electric field crests it then declines and the magnetic field crests and then declines and so forth and you have this self-propagating self-contained disturbance that once created can travel until it's received so what is electricity and magnetism describing? it's really describing the motion and behavior of electromagnetic waves from one place to another and those four equations together tell you that that's the picture that's really going on electric fields and magnetic fields all individually play a role in this but it should be possible to create a self-propagating wave that can be sent out from one location and then received at another by some kind of electromagnetic device Heinrich Hertz as in the Hertz was the first to satisfactorily demonstrate the existence of electromagnetic waves he basically took an inductor capacitor circuit so that what he could do was create a time-changing magnetic field by charging up the inductor and then the capacitor would cause a discharge from the inductor and then he had a coil someplace else and he could show that the time-changing magnetic field on one side of the room induced a response in the coil on the other side of the room it was Marconi, Guillermo Marconi who lived from 1874 to 1937 he was an Italian inventor and he developed the very first radio telegraph system which he demonstrated in 1894 wireless transmission of information the foundation of our entire society I mean, look at what it's done to the internet Robert Hire does anyone know who that is? Is that a familiar name? Yeah, I know you can read Did anyone know that before reading my slide? Yeah, okay, good, so you're all indoctrinated then Excellent He lived from 1860 to 1929 He was a physicist How many of you knew that? Yeah, I didn't think so That's not something they just go around selling at SMU He was actually the first American to communicate using electromagnetic waves Oh, look at that, it was in 1894 the same year that Marconi demonstrated the wireless telegraph system Hire attended a lecture by Hertz I believe it was Harvard got the idea that, oh, well I should build a device and use it to transmit a signal and so if I remember the story correctly he transmitted a signal from his laboratory at Southwestern University in Georgetown, Texas to the nearby jail So that was the first wireless transmission in the United States This is a picture of him Here's Hire demonstrating to other faculty at Southwestern University the use of X-rays So he was a real practicing physicist before he became a founder and first president of Southern Methodist University Now, like any good wave these waves travel at a speed What is that speed? Well, Maxwell could figure that out He could ask his equation well, how fast are these waves traveling And the answer he got was that the speed of an electromagnetic wave is always the same speed no matter what in free space regardless of how you were moving relative to it or not And this was the answer So, what's the number? Anyone want to punch that in their calculators? No Are you going to make me do it? Anyone want to volunteer from the back there? I know the answer It's 2 pi It's always 2 pi before you do And you can just give me the number Don't worry about the units too much Let's count zero If you take two non-zero numbers that are real and multiply them I don't think you get zero Are you going to zero? Wow, you guys need better calculators Something doesn't feel right about that I'm at 2.99 and it's 10 to the 8 2.99 times 10 to the 8 and I'll give you a clue for speed The units on that are meters per second So I'd probably be the square root not when a ride or something like that in your calculator because you're off by an order of magnitude But anyway That's the correct multiplication And what does that number look like? The speed of light And it was at this time that people realized what light was Light is an electromagnetic wave that travels from the source to the receiver at a fixed speed of 2.998 times 10 to the 8 meters per second Maxwell's equations, oddly enough said it doesn't matter how fast you are traveling relative to the to the source You will always measure the speed of light to be 2.998 times 10 to the 8 meters per second and that's weird And that is what initiated That's what initiated the following revolutions So This is just to illustrate the wave nature of light So this is a very simple experiment that was done by Newton originally who originally developed these optics and put around a split light into its spectrum So you can take white light you can send it through this thing called a prism which is just a carved piece of glass and different bands of color will appear on the other side And this is because the white light is made of a series of frequencies of light which corresponds to the color and those colors can be separated in the glass We now understand that that's because different frequencies travel at different speeds inside the glass So in material, the speed of light has changed It's slower than it is in free space but it can be calculated and that's sort of the basis of geometric optics which is what we're going to do next So this picture summarizes the things that you're going to be doing next Light comes in, it reflects some of it refracts on the surface of the glass and then hits the other surface and when it comes out you can have these effects where you split the bands of color These kinds of effects are really unfortunate in camera lenses You get things like chromatic aberrations where if your lens is not properly corrected it will split the colors before sending them to the film and you'll get all kinds of distortions in your picture as a result So understanding optics not only gives you an understanding, for instance, of a human eye which is essentially a lens system but also how one builds a better camera, how one builds a better laser, things like that Now, I mentioned earlier that humans are sensitive to what we call color We refer to color Color is here It's visible wavelengths Different colors of light, different frequencies different wavelengths correspond to different energies in the electromagnetic spectrum We refer to light that's between 700 nanometers which we call red to 440 nanometers which we call purple or violet That visible spectrum is compressed here Right here that's what we are exposed to as organic creatures from our particular species There are creatures that can see infrared Infrared are subred wavelengths We use infrared, for instance Well, this is actually wireless transmission but for TV remotes TV remotes use infrared Fun fact Most cameras the charge coupled devices, CCDs that make up the camera sensor are sensitive to the infrared So you can take a webcam aim your TV remote at it, push the button and you'll see the light come on in the infrared remote Your eyes can't see if the camera can and the camera converts it into visible wavelengths for you Okay So when you're cooking food or when you're getting strip naked by the TSA in a remote room this is how they're doing it They're exposing your body to microwaves That's one way of doing it at least Pretty harmless Your phones give out microwaves You cook food with microwaves at the right frequency and make water molecules vibrate That's not the frequency your phones use And you have the TSA loves this now so they can see metal on your body Down here we have the broadcast and wireless spectrum So radio Nice radio with an antenna That's long wavelengths, long antenna Those are used to receive music, audio broadcast propaganda, you name it Ultraviolet, now we're getting into higher energies, we can't see ultraviolet but this is high energy stuff This is the stuff that there are bands of ultraviolet that penetrate the earth's atmosphere make it down here and tan your skin They also interestingly break chemical bonds and cause cancer So these are high energy nasties You want to be careful of ultraviolet X-ray, you don't want to mess with X-ray It's great for seeing bones and getting through skin and stuff And it's also great for stripping your naked which is what the TSA also uses X-rays for to see if you have metal hidden on your body There's soft X-ray and hard X-ray They're both bad for you There's just lower energy X-rays and higher energy X-rays You really don't want to hit too much by either of those And then up here you have gamma rays which are just the do not disturb of the electromagnetic radiation That's bad stuff, but there's all kinds of stuff in the universe that makes gamma rays and PET imaging, actually where you use matter and antimatter in the body This creates a pair of gamma rays and you can look at the pair of gamma rays and figure out where in the body it came from You just have to be careful of how much you get That's what we see And this is what the universe is made of There's a lot more going on in the universe than our eyes and certainly our ears and sun waves can tell us and we've learned over the last century or so to begin to listen to this stuff Listen in the sense that we are looking at the universe in electromagnetic radiation Alright, so here's a question If water travels If water waves travel in water that is, they're a displacement of a medium and sound waves travel in air that is, they're a displacement of a medium air Then what the heck does light travel in? Because Maxwell's equations describe in free space There's no matter there No charge, no electrons There's nothing So what the heck are the electromagnetic waves traveling through to propagate from point A to point B This was a question that really bug people about the laws of electromagnetism They didn't seem to require a medium for these waves to travel and that flew in the face of all understanding of the mechanical universe So it was proposed that there was in fact a medium and it just wasn't in Maxwell's equations and Maxwell's equations were the new kids on the block and they're probably incomplete and wrong They called this medium the ether and physicists set out to find it and they did very sensitive experiments that absolutely was going to detect the existence of the ether causing light to slow down in some directions versus others as we move through the ether and they saw nothing They saw absolutely nothing So there was actually an experiment to see if the earth is moving through the ether in the universe there was a prediction about how that would affect light waves and they did the measurement, it's called the Michelson-Morley experiment they did it over and over and over again and it all went well beyond the sensitivity required to see this ether and then I thought, okay, well what if the earth is rotating is dragging the ether with it so then people used telescopes to see how light would be deviated depending on how the ether is dragging light with it as it, it saw nothing it saw none of this So there were repeated ways of testing for the presence of the ether and none of them ever observed it And so if Maxwell's equations require no medium the propagation of this electromagnetic wave and yet all the laws of Newton and all the greats that came before Maxwell and Faraday and Ampere and all these people required a medium, who's wrong? And it was this guy, Albert Einstein who lived from 1879 to 1955 who in 1905 published three papers one on the theory of atoms the atomic theory one on the nature of light and one reinterpreting space and time based on the theory of electromagnetism So Einstein took the radical idea that electromagnetism has the correct description of space and time and motion and that it's Newton's laws that are incomplete that is not what most people were doing, okay So he ran with that, he said okay well look, the laws of electromagnetism don't require a medium and if you observe that the speed of light is the same regardless of your state of motion that's the way the universe really is and all this business about if I travel if I run at light according to Newton I should see light speed up as it runs toward me in the same way that when you're heading toward a car at 60 miles an hour and you speed up to 75 to go at it faster it appears to be moving at you much faster than it was before light doesn't do that light always travels relative to you at 2.998 times 10 to the 8th meters per second regardless of your state of motion and that is a weird truth it is the truth so Newton had it wrong Galileo and his understanding of space and time was incorrect according to Einstein, electromagnetism has the correct description of space and time and he ran with it and so as a result of this we have a completely reimagined universe which is observation we've been held up for a century Einstein reimagined space and time, Newton and Galileo and all their colleagues and people that built on their work they just assumed that space and time were a fixed frame of reference in which all events happen imagine an invisible grid that fills the universe and for all observers that grid is the same space and time are the same for all observers Einstein said, well no, Maxwell's equations tell us that all observers definitely agree on events happening in space and time but they disagree on why they happen some people will think they happen because space is contracted some people will think they happen because clocks are running slowly for some observers space and time are different for different observers moving at different speeds that is a radical idea that has also been held up experimentally over and over and over again Einstein also recognized that there were some experiments that suggested that yes, lights a wave but you can't explain the outcome of some experiments by assuming it is only a wave if you assume it sometimes a particle like a little blob of energy that scatters off of something that better explains some experiments out there like the photoelectric effect I can demonstrate the photoelectric effect people would be interested in that in the last lecture the laws of electromagnetism describe light as a wave, not a particle Einstein proposed however that under certain conditions light may have a particle behavior as well and that culminates in this experiment called the photoelectric effect and these concepts launched twin revolutions relativity which was a general theory of space and time and quantum physics a general theory of matter and forces those were united in what we now call the standard model of particle physics which is a stupid name for something which describes everything we've ever been able to do in the laboratory with matter and forces okay but that when you put these two together okay at least special relativity motion space and time and quantum physics you get the thing that we're testing at the Large Hadron Collider to see if we can make it break for a change so if you'd like more I encourage you to go watch the first two episodes of the fabric of the cosmos which is hosted by physicist Brian Greene from Columbia University you see here the idea that time is relative it can be warped and stretched like a fabric the first episode is when is space and the second episode is the illusion of time you can kind of see where these are going so if you're interested in this topic go check these out they're each about an hour and enjoy and then the last thing I just yeah pbs.org and then the last thing I wanted the last thing I wanted to show you is this I love this because it culminates everything that we've been doing for the whole semester so if I turn this on to dynamo no power right no sound coming out of this alright probably should have tuned to the station that actually comes in there we go perfect so I can use mechanical work I can use an electromagnetic field that charges the EMF device the battery that's in the back of this and then once that battery is a little bit charged it can now receive and amplify electromagnetic waves coming in from this antenna so electricity, magnetism, magnetic induction inducing electromagnetic radiation and receiving electromagnetic radiation all nicely held in this tiny little plastic box so