32. Electromagnetic Waves
What is an Electromagnetic Wave?
What is an Electromagnetic Wave?
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Hey, guys, in this video, we're going to start talking about light and electromagnetic waves. Okay, So first we want to start by asking and answering the question. What is an electromagnetic wave? Exactly? All right, let's get to it. An electromagnetic wave, which is what light is made out of, is composed of oscillating electric and magnetic fields. Okay, so that's basically what, like is light is just a wave formed of oscillating electric and magnetic fields. I have a picture right here off one of the most common representations of an electromagnetic wave, which is a new electric field. Oscillating sign the soil early in the X direction. A magnetic field oscillation signing soil early in the Y direction and the whole thing propagating in the Z direction. You can see the velocity right here propagating in the Z direction. Okay, Now, at what speed does this like? Travel speed is a very important thing to know for a wave. If you want to relate things like the wavelength of the wave to the frequency of the wave, the speed of light in a vacuum is given by this equation. One over the square root off the vacuum permitted Vitti in the vacuum permeability and it's three times 10 to the eight meters per second. Okay, now this is a fundamental constant of the universe. The speed of light in a vacuum is constant. Okay, When light travels in a vacuum, it always travels at this speed three times 10 to the admits for second. Now, when it enters air, when it enters water, glass, oil, any other kind of medium light will travel at a different speed. And that's beacon change, depending on the color of light. And I'm sorry, the properties of the media, but in a vacuum, it is always see Okay, Always see three times 10 to the 8 m per second. Typically in a medium, you'll write the speed of light like I did here with a V. Okay, let's do a quick example. Blue light, with a wavelength of 450 nanometers, travels through a medium where the permeability of the permitted Vitti is four times the vacuum permitted Vitti and the permeability is just the vacuum permeability. What is the speed of the blue light in this medium? Okay, The speed of the light in any medium is actually the exact same equation. Except you put whatever the permitted Vitti and permeability are in that medium instead of in the vacuum. So this is gonna be one overall I'm gonna substitute. Are these two things given right here? So this is four. Absolutely not. You're not. I can pull that four out of the square root as a to And if you notice what I have right here, this one over the spirit of absolute not Mu not is just the speed of light. And I still have it two in the denominator. So this is just gonna be the speed of light divided by two, which is 1.5 times 10 to the eight meters per second. Okay, we did not have to know what four times the permitted it. Iwas right. We know that permitted Vitti is 8.85 times 10 to the negative 12. The permeability is four pi times 10 to the negative seven. Those were just Constance, but we don't actually have to plug them in. Okay, This is a proportionality problems. So all we have to do is work the proportions. All right now, electromagnetic waves satisfy the following three properties. They are always going to be trans verse waves. Okay, remember that longitudinal waves propagate along the oscillation direction. Trans verse waves propagate perpendicular to the oscillation direction. If you remember in the figure above, we had oscillating electric fields in the X direction oscillating magnetic fields in the Y direction and the propagation was in the Z direction. Obviously, the Z direction is perpendicular to both the X direction where the electric field oscillates and the Y direction where the magnetic field oscillates. So, yes, this is a trans verse wave. Okay. The speed of light actually defines the ratio between the amplitude or that maximum electric field and the amplitude or the maximum magnetic field where e this maximum electric field or the amplitude of those electric field oscillations equals c the speed of light times be so this ratio is always gonna be true that C equals e Overbey. If I just divide this be over, that ratio is always going to equal the speed of light regardless of whether it's in a vacuum, as I showed here. Or it's in a medium. Okay, So keep that in mind that the maximum electric field divided by the maximum magnetic field always equals the speed of light in the medium that it's in or in a vacuum if it's not in the medium, okay? And lastly, unique, completely unique. Thio, electromagnetic waves or light electromagnetic waves do not need a medium to propagate. This is entirely unique to electromagnetic waves, and this was actually a very, very important thing that was proven at the end of the hundreds early 19 hundreds. That led to a lot of brand new physics being done. Okay, they used to assume that light propagated in a medium called ether. They prove that that didn't exist. So now we know that light can travel in a vacuum. It's the only wave that can sound, cannot. Mechanical waves cannot Onley light. Alright, guys, that wraps up our intro into what exactly electromagnetic waves are. Thanks for watching
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