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Ch 34: Geometric OpticsWorksheetSee all chapters
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Ray Nature Of Light
Reflection Of Light
Refraction Of Light
Total Internal Reflection
Ray Diagrams For Mirrors
Mirror Equation
Refraction At Spherical Surfaces
Ray Diagrams For Lenses
Thin Lens And Lens Maker Equations

Concept #1: Ray Nature of Light


Hey guys in this video we're going to talk about something referred to as the ray nature of light describing light as a ray instead of a wave, let's get to it. Now light as we know is composed of electromagnetic waves, 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 rays which are individual lines that point perpendicular to those wavefronts, consider the image above me. I've drawn a moving light wave a propagating electromagnetic wave as a series of green wave fronts those wavefronts 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 wavefronts. Now light is always going to travel in a straight line when in a single media, whether it's air or water. I wrote a vacuum but vacuum is technically the absence of a medium light being the only wave that can propagate in the absence of a medium however when light is disturbed when it crosses a boundary between media then interesting things happen, they may not be interesting to you but they're interesting to physicists and so you're forced to learn them. These are in particular refraction and diffraction which are going to be two things that we talk about while discussing light in optics. In order to understand these two phenomenon we have to understand something called Huygens principle. Now Huygens principle makes two points the first point is that all points on a wave front act as point sources for spherical wavelets. So if I were to draw a wave front then every single point on that wave front is going to be producing these smaller waves which are called wavelets and the second point is that new wave fronts so new in time where is the wave going to be after some amount of time new wave fronts are formed by the tangent line across the apex of the wavelets that were produced by the last wave front. This is a lot of information but Huygens principle is fairly simple in application. Let's do just a little bit of application so we can see what exactly it's saying so on the left I have a wave front produced by light that is moving in a single direction this light is what would be called collimated light, because if I were to draw rays for this light right all the rays have to be perpendicular to the front those rays are all parallel to one another so collimated light is light whose rays are all parallel to one another so if I were to choose a point on this wave front you'll see that these little wave lids right are being produced and they travel some distance in some amount of time. The distance I chose is just the wave length because that's where the next wave front is going to be so that's one point I can choose another point on the wave front and it will also produce these spherical wavelets and a third point on the wave front and also these spherical wavelets are going to be produced. Now what Huygens principle 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 wavelengths. So here's the apex for the red wavelet, here is the apex for the green wave, here is the apex for the black wavelet and our new wave front is just going to be tangent to those apexes this is the new front and as you can see the light doesn't change direction light 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 alright let me minimize myself for the next thing. According to the old wave front we have a light that's moving in all manner of different directions it's actually moving at all possible directions it's moving spherically and this kind of light we would call isotropic the same in all directions alright now if I choose a point on this wave front you can see that these spherical waves are emitted and their emitted for a distance of the wavelength right the next wave front is going to appear one wavelength away. Right I choose another point spherical wavelets come out travel a distance of the wave length, choose a third point. Spherical wavelets come out. Traveling a distance of one wavelength now we want to mark the apexes on all of these waves. Because those apexes are going to determine where the next wave front appears and that wave front has to be tangent sorry tangent to all of those apexes, 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.

Let's do an example, we want to explain light reflecting off of a mirror using Huygens principle for this I'm going to draw wavefronts. I have the light ray I want to draw a 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 wave front is here then it travels a wavelength and now it's here then it travels some more and it's here and then it's here and then it's here and then it's here and then it's here. So where did the wave front or where did the electromagnetic wave first contact the mirror? It first contacted the mirror here. Where did it contact the mirror first where did it contact the mirror second? It contacted the mirror second right here and where did it contact the mirror last? It contacted the mirror last right here. That means that that third point of contact is going to have the least amount of time for the wavelet to propagate right because it occurred last so it's wavelet is going to be the smallest. The second point is going to have an intermediate wavelength because it occurred seconds so it has the second largest amount of time to propagate so I'll draw the wave like this now the first point of contact occurred the earliest it occurred the longest amount of time ago so it's wavelet is going to be the largest because it had the most time to propagate so its wavelet is going to look something like this 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 wavelet is small my green wavelet is intermediate my blue wave lit is large and now where are the apexes remember that the apexes 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 going to draw a line like this. If this had been done properly with computers and everything that line would be perfectly tangent to all of the wave fronts sorry all the wavelets 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 and that is how you explain reflection using Huygens principle. Alright guys that wraps up our discussion on the ray nature of life. Thanks for watching.