b tech applied science unit 1 physics and this video is about wave interference consider this we have a speaker which is making a noise uh it's not a very interesting noise it's just a single note one single frequency one wavelength might be doing a very boring single note and here is an observer who can hear the noise listening to the noise now what would happen if we had two speakers now uh two coherent sources that basically means that the two speakers are doing exactly the same thing they have the same wavelength they're emitting the same note they've got the same amplitude and when they leave the speakers the waves are in phase with each other i'll talk more about that in a bit so two coherent sources the two speakers are doing the same thing at the same time now will the noise heard by the observer will it be louder and one would imagine that it should be if there's two speakers instead of one however in some places the noise is louder but in some places the noise is quieter so what's happening and we can explain this the principle of superposition when waves from two sources arrive at the same place then they add together now they may add together if they arrive in phase then we get constructive interference if they arrive in antiphase then we get destructive interference in phase means they go up and down together these two kids are on swings here and they're swinging together they are in phase with each other so if the two waves arrive in phase then we get constructive interference and we end up with a big wave look at these two diagrams here waves arriving from source 1 waves arriving from source 2 they are in phase they are going up and down together at the same time and the result of that is we get a big wave so the observer would hear a loud sound if the waves arrive in phase then they interfere constructively here waves from source one waves from source two they are in antiphase they're doing the opposite when one's up the other one's down they are in antiphase and when these two waves meet they will cancel each other out and will get very very little if the waves arrive in antiphase then they interfere destructively destructive interference here's another example of a type of wave this is water waves and a dripping tap is acting as a point source and it's producing these circular water waves circular wave fronts going outwards now here we have two uh point sources and they are coherent when the waves first start at the point that they start they have the same amplitude they have the same frequency and they are in phase they are coherent sources and these two waves will travel outwards and what we're interested in is what happens when they meet will they add together or will they cancel each other out okay first of all the wavelength of the wave the circles are wave fronts and the distance between the wave fronts is a wavelength you can think of them as the the crest of the wave yeah the lighter parts are the crest of the wave although not necessarily okay now what's happening is that in certain places they will add together at point x they will add together we will get constructive interference and at point zed they will add together we will get constructive interference at point y they will cancel each other out that will be destructive interference so again at some points they will add together constructively and at some points we will get destructive interference and they will cancel each other out okay now whether they arrive in phase or antiphase depends on something called the path difference and the path difference is the difference in how far they have traveled if you look at point x and you look at the red line going through point x the waves that arrive anywhere on that line will arrive in phase and i know that they will arrive in phase because they have traveled the same distance if they started off going up and down together they've traveled the same distance in the same amount of time so they will arrive in phase we will get constructive interference anywhere along that line now look at point zed they will arrive in phase as well they haven't traveled the same distance but the difference between the distances the path difference is actually equal to and can you guess what it is yes the path difference is a wavelength so maybe one of them has traveled seven wavelengths and the other one has traveled eight wavelengths so the path difference is a wavelength and they will arrive in phase for point y where we get destructive interference anywhere along that green line the path difference will be half a wavelength