Ray Nature of Light

by Patrick Ford
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Hey, guys. In this video, we're gonna talk about something referred to as the ray nature of light describing light as a ray instead of a wave. Okay, let's get to it now. Light, as we know, is composed of electromagnetic waves. Okay, all waves have things called wave fronts, which is a point of maximum oscillation for that wave. In the case of light, it's a point of maximum electric field. Because of this, it's often convenient to describe these moving electromagnetic waves as just raise, which are individual lines that point perpendicular to those wave fronts. Okay, consider the image above me. I've drawn a moving light wave of propagating electromagnetic wave as a Siris of green wave fronts. Those wave fronts, which are points of maximum electric field right there peaks have to be separated by the wavelength, right? The peak to peak distance is just the wavelength, and the rays that are going to be drawn have to be drawn so that they're always perpendicular, no matter where they are to those wave fronts. Okay, now light is always going to travel in a straight line wind in a single medium. Okay, whether it's air or water. I wrote a vacuum, but vacuum is technically the absence of a medium like being the only wave that can propagate in the absence of medium. However, when lights disturbed when it crosses a boundary between media, then interesting things happen. Okay, They may not be interesting to you, but they're interesting to physicists. And so you're forced to learn them. Thes are in particular refraction and diffraction, which are gonna be two things that we talk about while discussing light and optics. Okay, In order to understand these two phenomena, we have to understand something called Hodgins Principal. Alright. Now Hagen's principle makes two points. The first point is that all points on a wave front act as point sources for spherical wave. Let's so if I were to draw away front than every single point on that wave front is going to be producing these smaller waves which are called wave lits. Okay. And the second point is that new wave fronts so new in time. Where is the wave gonna be after some amount of time? Knew what wave fronts were formed by the tangent line across the apex of the wave. Let's that were produced by the last wave front. Okay, this is a lot of information, but Hagen's principle is fairly simple and application. Okay, let's do just a little bit of applications so we can see what exactly it's saying. So on the left, I have a wavefront produced by light that is moving in a single direction. Okay, this light is what would be called Qala mated light. Okay, Culminated. Because if I were to draw, raise for this light, right, all the rays have to be perpendicular to the front. Those rays are all parallel toe. One another column aided light is light whose rays are all parallel to one another. So if I were to choose a points on this wave front, you'll see that these little wave lits right are being produced and they travel some distance in some amount of time. The distance I chose is just the wavelength, because that's where the next wave front is gonna be. So that's one point they can I can choose another point on the way front, and it will also produce these spherical wave. Let's okay, and the third point on the way front. And also these spherical weight. Let's are gonna be produced now. What Hagen's principal says is that the next wave front right located one wavelength away is going to be produced perpendicular. Sorry tangent to the apex of each of these wave. Let's Okay, so here's the apex for the red wave lit. Here's the apex for the green wave lit. Here's the apex for the Black Wave lit, and our new way front is just going to be tangent to those Apex is this is the new front, and as you can see, the light doesn't change. Direction like that was moving to the right is still moving to the right, right? All of those rays are still pointing in the same direction as they should, because in a single medium, light shouldn't change direction. All right, let me minimize myself for the next thing. According to the old way front, we have light that's moving in all manner of different directions. Okay, it's actually moving in all possible directions. It's moving spiritually and this kind of light we would call Aissa Tropic the same in all directions. All right, now, if I choose a point on this wave front, you can see that this spherical waves a remitted and they're omitted for a distance of the wavelength. Right. The next wave front is gonna appear one wavelength away. Right? I choose another point. Spherical wave. Let's come out. Travel a distance of the wavelength. Choose a third point spherical wave. Let's come out traveling a distance of one wavelength. Okay, Now I want to mark the apex is on all of these waves because those apex is air gonna determine where the next wave front appears And that way front has to be tangents. Sorry, my hand got a little sweetly there Tangents to all of those. Apex is okay, So it's going to be another spherical wavefront. And you'll see that once again, the light continues traveling in a straight direction because it's still perpendicular at all those points on the new wave front. And that is to be expected. Light should continue to travel in a straight direction unless it moves into a new medium. Okay, so let's do an example. We want to explain light reflecting off of the mirror using Hagen's principle. Okay. For this, I'm gonna draw wave fronts. Okay? I have the light Ray, I want to draw wave fronts. Each new wave front I draw is going to be at a new point in time. Right? This wave is traveling. So at one point in time, the way front is here. Then it travels a wavelength. And now it's here. Then it travels some or and it's here. And then it's here. And then it's here. And then it's here. And then it's here. Okay, So where did the wave front or where did the electromagnetic wave first contact the mirror? It first contacted the mirror here. Okay, Where did it contact the mirror? It's alright. First, where did it contact the mirror? Second, it contacted the mere second right here. And where did it contact? The mirror last contacted The mirror Last right here. Okay. That means that that third point of contact is gonna have the least amount of time for the wavelet to propagate because it occurred last. So it's wavelet is gonna be the smallest. The second point is gonna have an intermediate wavelength because it occurred second, So it has the second largest amount of time to propagate. So I'll draw the wavelet like this now. The first point of contact occurred the earliest it occurred the longest amount of time ago. So it's wavelet is gonna be the largest because it had the most time to propagate. So it's wavelet is gonna look something like this. Okay. And now I'm going to redraw this scenario because that picture is already getting complicated and I want to see what the new wave fronts look like. So my blue point is here. My green point is here. My red point is here. My red wave small. My green wavelet is intermediate. My blue wave lit is large. Okay? And now where are the apex is? Remember that the apex is tell us where that new wave front is going to be. There is an apex. There's an apex, There's an apex. And I'm just gonna draw a line like this. Okay, if this had been done properly with computers and everything, that line would be perfectly tangent to all of the wavefront. Sorry, all the wave. Let's But I'm a person, so I can't draw things perfectly. But the line looks something like that. Which means that my new ray should look like this and because it moves in a straight line, the new wave fronts propagate like this. And this is what my new light ray looks like. Okay. And that is how you explain reflection using Hagen's principle. Alright, guys, that wraps up our discussion on the ray nature of light. Thanks for watching.