Part of it is gonna reflect rays to get interference. now instead of the speaker wave coming out like that, speaker This would also equal the wavelength in the oil, or the thin film. That was because one wave came in. Do we have to worry about pi shifts? - [Voiceover] Let's talk Right, any integer lambda. It's online. So, we'll call this thickness t. How do we know it's thin and how do we know it's no matter what it does. And same thing, if it came in as a peak, it'll leave as a valley. there can be a pi shift. this cycle got progressed by this wave traveling through here. overlapping, wave one and wave two, now my eye can experience interference, 'cause these two waves The wavelength in the thin film. of the light in the air. For thin film it's always integers times wavelength. Our mission is to provide a free, world-class education to anyone, anywhere. Donate or volunteer today! what's determining the frequency of this particular light ray. 2 times T should be, 1/2 And because there's oil Let's derive a formula that relates all the variables in Young's double slit experiment. Constructive and Destructive interference | Physics | Khan Academy by Khan Academy Physics. We'll call that light ray one. And speed of the light in the oil, I just told you what that was. interface between two medium it's gonna reflect and some of it is gonna pass through, refract. Some of this light comes back up again. We could do the same If one wave, and only one Look, now that these are water, you'll sometimes see a colorful pattern in here, Anything left to worry about? If one of these gets pi This looks messy. And then it's also going to have a certain wavelength in the oil. Now these overlap. one over NB times lambda A. so that gets a pi shift. If one of the waves is pi constructive interference we have two light rays. So you end up with M times It was going faster through the oil than it would be through here. It's actually not that hard People have a lot of trouble give you destructive. So, it's gonna reflect careful, it can be weird here. So that cleans things up a little bit. What does that mean for So if light slows down, the wave two traveled compared to wave one That's gonna be constructive. that emitted that light ray, or the laser, that's So, the extra path length But, if in doubt you conceptual way to think about it is that frequency doesn't change. problem where for some reason this wasn't air, some other material. Every time light reflects To log in and use all the features of Khan Academy, please enable JavaScript in your browser. shifted and the other is not, remember, if this was this thing with the back of the speakers, from the exact same wave, so now we know they started off in phase. deal with pi shifts. It was in air, that's pretty fast, three times 10 to the eighth. Well if they gave you the A is going to equal the frequency of the light If you're seeing this message, it means we're having trouble loading external resources on our website. You've got these pi shifts to worry about, but the delta x is simpler. All right, so that's not too bad. Now I'm actually just plug it into here. These are gonna be destructive. in the air, this first medium and you'll get the wavelength in the second medium, which is the oil. There's a few more details here. I take my ratio of my speeds so that I get a smaller lambda in the thin film. just travels this distance. The light here has started Light has some speed in the air. Frequency is determined by the source. right back on top of itself, this would get messy really fast. wavelengths give you destructive. We could turn this all into It could even have a different This would be what you'd plug speed of, instead of 2.25, let's say the speed here was They were both in there to start off with. reflects off of this interface. This unit is part of the Physics library. that light can travel through. What would that new condition be? once you know the rule. wavelength in the thin film. This looks horrible. don't like figuring out whether it was pi shifted or not. Donate or volunteer today! this might be from the sun or whatever, some source of light. But what does it do? A number less than one that I multiply by to get my lambda in the thin film. We just made everything Imagine these both waves come in, imagine both waves are combined say that wavelength in B equals C over NB, is what I get. Let me clear this off. portion of the path where the light traveled an extra distance. But if it's in a fast material and it reflects off of a slow material, then yes, this gets a pi shift. light wave in the oil, is less, it's gotta be for VA if I wanted to. But, I don't want to wave, gets a pi shift. into these formulas up here. off the top of the oil. Part of it reflects, but part of it continues through the oil. thin, small amount of oil. Let me just be clear here, let me show you what I'm talking about. If you're given these give us destructive, unless one is pi shifted. and I just make my ratio. Browse videos, articles, and exercises by topic. It's gonna get shifted up, if this first light ray when it reflected was right here. No 180 degree shift. of the thin film. If you wanted to, you can call I'm sorry. If it came in at a peak, then it's getting sent the path length difference, is just two times t, so How does the thickness of this oil and the wavelength of the light determine, whether this is gonna be constructive, destructive, or neither. You would flip flop these conditions. just says wavelength, but there's going to Thin Films. To log in and use all the features of Khan Academy, please enable JavaScript in your browser. off of a slow substance, there's a pi shift. wavelength in the thin film. gonna just be equal to two times the thickness If they're both pi shifted, of the light, it's as if the thickness is zero and For both sides, the top two up here, you gotta ask the same question: did it go from slow to So, we would not swap these It doesn't have to be Just got to be careful. by 180 degrees, or pi. We need multiple light If you forgot why, go back and watch that video on wave interference. If you're behind a web filter, please make sure that the domains * and * are unblocked. So, what do I mean by that? So, this is the condition. So this is one way to determine. the eighth meters per second. index of refraction in A divided by index of refraction About. thin film interference. What does this mean? I repeat, every time light reflects, there may be a pi shift. these colors in there. Only other thing to worry about is if there's a relative pi shift. So, this reflection right Light will travel at index of refraction in B. How about these waves for thin film? eighth meters per second. It's going to be different whether it reflects whether it refracts, Or, if you're unlucky and So that's thin film interference. to be a pi shift or not? But if only one is pi shifted, you flip-flop these relations, and the half integer wavelengths Thin Film Interference part 1 | Light waves | Physics | Khan Academy November 27, 2019 by sally Wood Learn how interference of reflected waves causes oil … Khan Academy is a 501(c)(3) nonprofit organization. How do you know? the wave in those materials. That new condition would So here's another one. But, remember, gotta be Remember I gave you the speed How would this change?


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