Diffraction - Online Tutor, Practice Problems & Exam Prep

Diffraction

Hey, guys. In this video, we're gonna talk about a phenomenon called diffraction. Alright? 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 2 media and that light could reflect off of the boundary or it could transmit through and refract its 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 a straight line, allows light to be described as rays. So just to refresh ourselves, we can 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 an, 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 wavefronts at all points. So I can draw 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 slit is a small opening between 2 barriers of light. Alright? Let me minimize myself. We have here just light traveling. I drew 3 hypothetical light waves each of which is a different color. Okay? Here I've indicated 2 2 boundaries and we're gonna imagine that these boundaries, these barriers are completely reflective. Okay? 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. 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's sort of a catchall term that refers to all phenomenon associated with light rays being spread apart when they encounter a slit. Okay? A slit between 2 barriers. Right? Diffraction isn't gonna occur for any slit. Okay? The slit's width I say the slit must be small, but what I mean is the width must be small compared to the wavelength of the light. Okay? 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 2 scenarios. Alright? I have light light of a particular wavelength encountering a slit of a particular width. And I've shown what happens when the wave fronts pass 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 wavefronts the rays before encountering the slit have to look like this. Okay? This by the way is referred to as collimated. This funky looking letter there is supposed to be an 'l'. Collimated light. Okay? Light that is all initially parallel to itself. All the rays are parallel. Okay? Now the wavefronts I have 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 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. Okay? But now choosing another hypothetical scenario. One where we have a larger wavelength and a significantly smaller hole. Now I wanna consider the scenario where the length is smaller than the width sorry, the width of the slit is smaller than the wavelength of the light. Okay? If I'm gonna 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. Okay? 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. 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 isotropic. And it's absolutely not collimated. Isotropic just means the same in all directions. Okay? Since the light intercollimated and exited isotropically, the light rays were disturbed. They did spread out and this is known as diffraction. Okay? Now something interesting happens when the light is allowed to diffract or when you allow for light diffracting. Okay? Light 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, the light that enters collimated will always leave collimated. Okay. That's what we mean by no diffraction. So what that means is when light is entering the slit collimated, it's all coming out collimated and you're gonna 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 that you can see. But if you allow diffraction then so long as the width of this 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. Alright. So dark bright. 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.