Diffraction

by Patrick Ford
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Hey, guys, In this video, we're gonna talk about a phenomenon called diffraction. All right, let's get to it. Remember that light travels in a straight line so long as it's not disturbed. Okay, we've seen one type of disruption of light before, when we saw light encountering a boundary between two media and that light could reflect off of the boundary. Or it could transmit through and refract it's angle as it passed into the second medium. Okay, this allows light. This fact that light travels so long as it's not disturbed in straight line allows light to be described as raise. So just to refresh ourselves, we condone draw any wave as successive wave fronts. Each of these wave fronts drawn in green is a point of maximum oscillation in the case for a, um, electromagnetic waves in the case for light. It's a maximum electric field, and we can draw raise such that they're perpendicular toe all the wave fronts at all points so I can draw raise like this, okay, and you can see clearly that it's perpendicular wherever you want to measure, okay. And the distance between wave fronts the distance between two peaks as we know is just the wavelength. That's the definition of the wavelength. Okay, Now, a common way to disturb light that we haven't talked about is for light to encounter a slit, Okay. And a slip is a small opening between two barriers of light. All right, let me remind myself, we have here just like traveling. I drew three hypothetical lightwaves, each of which is a different color. Okay, here, I've indicated to boundaries, and we're gonna imagine that these boundaries these barriers are completely reflective. Okay? Or not reflective at all, but not Trans missive. They completely block out any transmission of light, all the light that's allowed to transmit, then the only light That's logic transmit is the one that passes through the slit. So the green light is the only light on the other side. Okay. Now, depending on the size of the slit, depending on the width of the slit, right this dimension the rays may or may not be disturbed. They don't have to be disturbed as they pass through. They may or may not be disturbed. Alright, What diffraction is is it sort of a catchall term that refers to all phenomenon associated with light rays being spread apart when they encounter a slip. Okay, a slip between two barriers. Right. Diffraction isn't gonna occur for any slit. The slits whipped. I say that slip must be small, but what I mean is the whip must be small compared to the wavelength of the light. Okay, So the fraction will Onley occur if this dimension right here is small compared to this dimension, which is the wavelength? All right, now let's see what diffraction looks like right here. I have two scenarios. All right? I have light of a particular wavelength encountering a slip of a particular width. And I've shown what happens when the wave fronts pass through that slit. Okay, so let's draw the rays and see if diffraction occurs here in order to be perpendicular at all. Points to the wave. Fronts to raise before encountering the slit have to look like this. Okay. This, by the way, is referred to as culminated this funky looking letter. They're supposed to be an l column. Mated light. Okay. Like that is all initially parallel to itself. All the razor parallel. Okay, Now the wave fronts I've shown passing through the slit what do the rays look like passing through the slit? Well, they still need to be parallel to one another in order to be perpendicular at all points on the way front. So it's culminated before passing through the slit and culminated after those rays never spread apart, their culminated entering their colonnaded exiting. That means that there was no diffraction here. Okay, but now choosing another hypothetical scenario, one where we have a larger wavelength and a significantly smaller hole. Now, I want to consider the scenario where the length is smaller than the width. Sorry. The width of the slip is smaller than the wavelength off the light. Okay, If I'm gonna draw the rays for this light, you can see that once again, it has to be column mated. That's the only way to match raise to those wave fronts. Okay, but the wave fronts look different coming out of the slit. Now, instead of them being parallel wave fronts, they're actually wave fronts that are moving spiritually outwards. Okay, So, in order to draw the rays, remember, it has to be perpendicular to everything. This is perpendicular, perpendicular, perpendicular, but at a different angle. I need to draw the ray at a different angle, right? So they point out equally in all directions. Okay, this is known as is a tropic, and it's absolutely not colonnaded. Okay, is a tropic just means the same in all directions since the light inter column aided and exited Aisa tropically the light rays were disturbed. They did spread out, and this is known as diffraction. Okay, Now something interesting happens when light is allowed to do fractal when you allow for light refracting Okay, Like passing through a slit acts differently if you ignore diffraction. So in the left figure, we're gonna pretend like diffraction isn't a thing, meaning that if we look at these two figures up here really quickly, no matter the relative size of the width of the slit to the wavelength of the light, that interest column mated will always leave. Column mated. Okay, that's what we mean by no diffraction. So what that means is when light is entering the slit, call the mated. It's all coming out culminated, and you're gonna get a single bright spot on some sort of screen behind the slip. That screen is just there to collect the light to allow the light to land on it so that you can see. But if you allow diffraction, then so long as the width of this slip. So this dimension that horizontal whip so long is that width is less than the wavelength of light. What's gonna happen is that initially, colonnaded light is gonna come out equal in all directions, and it turns out that you don't get a continuous band of bright light. You actually get alternating bits of bright light and dark light, right, So dark, bright, Okay. And this alternating pattern of bright and dark spots of light is known as a diffraction pattern. Okay. And it's unique to the particular diffraction situation that the light is in. Okay, this wraps up our introduction on diffraction. Thanks for watching guys.