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Current time:0:00Total duration:11:02

Video transcript

you might know that if you've got a positive charge sitting out in space it's going to create an electric field and that electric field is going to point radially outward away from the positive charge and you might know that if you've got a current in a wire that current is going to create a magnetic field and that magnetic field is going to loop around that wire it's going to look something like this but people started to realize there's another way to create a magnetic field besides having a current and there's another way to create an electric field besides having charges turns out if you have a changing electric field in some region of space even if that region has no current in it by the fact that it has a changing electric field that's going to induce or create a magnetic field so here was a way to create a magnetic field that didn't even require a current it just required a changing electric field in that region and people also started to realize if you've got a changing magnetic field that that is going to create a changing electric field and a very clever Scottish physicist named James Clerk Maxwell realized hold on if this is true if a changing electric field can create a changing magnetic field then a changing magnetic field could in turn create a changing electric field which could in turn create a changing magnetic field and you can set up some sort of chain reaction here that can propagate like a wave and you would get a wave of electric and magnetic fields that can travel outward and have sort of an independent existence now from whatever parent current or parent charge created them so how would you create a changing electric field you can take this charge and sort of wiggle it upside down up and down sounds like an antenna that's what you could do in an antenna that would create a changing electric field and then a changing magnetic field and this could propagate outward and similarly you can have a current how could you create a changing magnetic field just have this current oscillate back and forth changing that would create a changing magnetic field which create a changing electric field and that could propagate outward and now this wave could have its own independent existence and keep moving forward at some speed and we call these electromagnetic waves because electro they have electric fields and magnetic because they have magnetic fields and waves because they propagate outward according to wave equations and similar to all the other waves that we've seen there is one difference these waves don't actually need a medium they could travel through straight vacuum you don't need actual particles of some stuff in here these can move through the vacuum which is a little weird because the only thing that's waving here the only thing that's oscillating is the value of the electric and magnetic fields and so these are electromagnetic waves this is a pretty horrible drawing of it looks a little bit more like this these yellow lines represent the magnetic fields these are directed up and down they point up and down they don't stick out up and down and you have to draw it that way but what I'm trying to say is at this point right here it's not that there's a magnetic field that sticks up it points up and I just draw it with a bigger vector to show that at this point right here along this axis this axis could be say the X direction so if this is the X direction then at this position in space there would be a relatively large magnetic field it would point up and this these red lines represent the electric field so at this point in space there'd be a relatively large electric field and the electric field is at a right angle to the magnetic field so these are perpendicular that's how this process has to happen it's going to create a electric and magnetic field that are perpendicular to each other so the electric field sticks out this is pointing think of it as out of your computer screen or your screen whatever you're watching this on and these electric fields back here point into the screen and so this is in 3d this is happening in 3d electric field oscillating into and out of your screen magnetic field oscillating up and down and the whole wave traveling to the right so this whole thing happens all three of these directions are at right angles so if I imagine a three dimensional axis here so XYZ this would be the direction of the speed of this or the velocity of this way if it doesn't have to point in this direction but whatever direction is pointing you'd have velocity in one direction you'd have magnetic field in the other direction and you'd have electric field in one more perpendicular direction all three of these are perpendicular to each other so the magnetic fields perpendicular to the electric field electric field perpendicular to the magnetic field and both the magnetic and electric field are perpendicular to the direction that the electromagnetic wave is traveling and the speed at which these waves travel is the speed of light C and by C I mean three times ten to the eighth meters per second because light is just an electromagnetic wave light is a special example one particular example of electromagnetic waves but it is only one example these waves can have any wavelength look from here to here this since this is in space the graph of the wave in a long of spatial direction this would represent the wavelength these waves could have any wavelength any particular wavelength and any particular frequency the frequency would be the rate at which these change so if you'd watch this in time this point in space would have a magnetic field that would point up then we point down there to point up and then a point down and the rate at which that's happening the number of times per second would be the frequency these waves could have any frequency but for one special region the region is the visible spectrum so we call the regional frequencies and wavelengths that electromagnetic waves can have the electromagnetic spectrum and there is a lot to learn about the electromagnetic spectrum let me just show you really quick if we pretend we're graphing here along this line higher frequency so let's say in that direction this is pointing higher frequencies to the right in other words higher Hertz since that's what we measure frequency in but if that's higher frequency because of this formula speed of a wave is lambda times frequency wavelength times frequency if the frequency increases the wavelength has to decrease so in this rightward direction this would be smaller wavelength and wavelengths measured in meters but when you're talking about light since you have such small wavelengths you often talk about nanometers so where would the visible spectrum be the visible spectrum would just be this little smudge here in our range this would be the this whole thing is called the electromagnetic spectrum so the electromagnetic spectrum and the visible spectrum is just right here this would be the visible spectrum so this is visible between red and violet and red is if I ask you what frequency that is that would be around four times ten to the fourteenth Hertz and violet would be around seven point five times ten to the fourteenth Hertz or if you wanted to talk about wavelength red would be around 750 nanometers and violet would be around 400 nanometers and by nano we mean ten to the negative ninth okay but this is just a small region you can have higher frequencies than violet and what do we call those we call those ultra violet and we know these are bad for us and they're bad for us because as we go to the right here higher frequency turns out this also means more energetic light so if you were to talk about the photons that make up this light if you're talking about a quantum mechanically those photons as you go forward higher up into the frequencies have more energy that means they're more dangerous and they're more dangerous because if they have more energy they can add that energy to your cells and break them part and caused damage and so this causes problems as you go further out this way so we put on sunscreen to protect ourselves from ultraviolet light but this isn't the highest frequency you can have much higher frequencies you can have x-rays x-rays are even higher frequency even more dangerous which is why x-ray technicians you go in there and they're like hey stand right here I'm going to get your x-ray while I hide behind this wall because if they had to sit there and take that x-ray every single day it would really be bad for them but this isn't even the highest you've got things called gamma rays and gamma rays are unbelievably energetic light and these are very dangerous this happens these come from space or from some sort of nuclear reaction these are extremely dangerous gamma rays that's why you always hear them in comic comic books when people want to blame like why some hero was created they say oh I was a gamma ray because it just sounds really powerful it won't turn people into superheroes but it's very energetic then down here you've got infrared so this would be below red you couldn't see this either it would be infrared and then below that you've got microwaves and microwaves in this region there's lots of useful stuff your cellphone's signals are in the microwave region TV signals that are sent through the air and the microwave region this is used for a lot of cases and then below this you've got radio waves even though technically speaking FM radio waves are more in the microwave region these are more like a.m. so when people started first making radio they used frequencies down here and that was the AM frequency range so this is the electromagnetic spectrum the visible range is a small region of it we can't see anything outside of that but these are extremely useful in a lot of different cases and dangerous so you need to know about the electromagnetic spectrum