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Ch 35: Wave OpticsWorksheetSee all chapters
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Diffraction with Huygen's Principle
Young's Double Slit Experiment
Single Slit Diffraction

Concept #1: Diffraction


Hey guys, in this video we're going to talk about a phenomenon called diffraction. Alright, let's get to it. Remember that light travels in a straight line and so long as it's not disturbed. We've seen one type of disruption of light before when we saw a 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. This allows light this fact that light travels as long as it's not disturbed in a straight line allows light to be described as rays so just to refresh ourselves, we can draw any wave as successive wavefronts. Each of these wavefronts drawn in the green is a point of maximum oscillation. In the case for an electromagnetic wave in the case for light, it's a maximum electric field and we can draw rays such that they're perpendicular to all the wave fronts at all points. So I can draw rays like this. And you can see clearly that it's perpendicular wherever you want to measure and the distance between wavefronts, the distance between two peaks, as we know is just the wavelength. That's the definition of the wavelength. Now a common way to disturb light that we haven't talked about is for a light to encounter a slit. And a slit is a small opening between two barriers of light alright I'm going to minimise myself. We have here just light travelling. I drew three hypothetical light waves, each of which is a different colour. Here I've indicated two boundaries and we're going to imagine that these boundaries, these barriers are completely reflective or not reflective at all but not transmissive. They completely block out any transmission of light. All the light that's allowed to transmit then, the only light that's allowed to transmit is the one that passes through the slit. So the green light is the only light on the other side. Now, depending on the size of the slit depending on the width of the slit, 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's sort of a catchall term that refers to all phenomenon associated with light rays being spread apart when they encounter a slit. A slit between two barriers. Diffraction isn't going to occur for any slit. The slit's width I said the slit must be small but what I mean is the width must be small compared to the wavelength of the light. So diffraction will only occur if this dimension right here is small compared to this dimension which is the wavelength alright. Now let's see what diffraction looks like. Right here I have two scenarios alright, I have light of a particular wavelength encountering a slit of a particular width and I have shown what happens when the wave fronts pass through that slit. So let's draw the rays and see if diffraction occurs here. In order to be perpendicular at all points to the wavefronts, the rays before encountering the slit have to look like this. This by the way is referred to as collimated. This funky looking letter there's supposed to be an L. Collimated light. Light that is all initially parallel to itself, all the rays are parallel. Now the wavefronts I've shown passing through the slit. What do the rays looks 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 wavefront. So it's collimated before passing through the slit and collimated after. Those rays never spread apart. They're collimated entering, they're collimated exiting. That means that there was no diffraction here. 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 slit is smaller than the wavelength of the lights. If I'm going to draw the rays for this light, you can see that once again, it has to be collimated. That's the only way to match rays to those wavefronts. But the wavefronts look different coming out of the slit, now instead of them being parallel wavefronts they're actually wavefronts that are moving spherically outwards. 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. This is known as isotropic and it's absolutely not collimated. Isotropic just means the same in all directions. Since the light entered collimated and exited it isotropically, the light rays were disturbed, they did spread it out and this is known as diffraction. Now something interesting happens when light is allowed to diffract or when you allow for light diffracting. Light passing through a slit acts differently if you ignore diffraction. So in the left figure we're going to 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, the light that enters collimated will always leave collimated. That's what I mean by no diffraction. So what that means is, when the light is entering the slit collimated, it's all coming out collimated and you're going to get a single bright spot on some sort of screen behind the slit. That screen is just there to collect the light to allow the light to land on it so you can see but if you allow diffraction then so long as the width of the slit so this dimension that horizontal width so long as that width is less than the wavelength of light, what's going to happen is that initially collimated light is going to 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. So dark, bright. And this alternating pattern of bright and dark spots of light is known as a diffraction pattern and it's unique to the particular diffraction situation that the light is in. This wraps up our introduction on diffraction. Thanks for watching guys.