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Current time:0:00Total duration:8:37

Video transcript

in a previous video we saw that if you take a concave mirror a concave mirror is a curved mirror in which the inner part is reflecting the inner side is reflecting and if you incident a paddle beam of light then these rays of light after reflection will get focused at a single point they will converge at a single point called as the focus of that mirror and you can get pal rays of light from any source which is far away something like the Sun or anything which is far away the rays of light from that can be assumed to be paddle in this video what we'll do is we'll look at some applications of this concave mirror where do we use them in our daily life all right so let's begin one of the applications we can see is in microscopes so if I just show you a compound microscope let me show you that a compound microscope over here you may or may not have used it but if you have then you then you know that you keep your slide whatever you want to observe or somewhere over here and you need that slide to be lit up otherwise you can't see it through the microscope isn't it and the way we light that slide up is by using a mirror over here and what this mirror does is that it takes in the Sun right and then focuses all that sunlight right at this light I don't think in this picture the mirror is aligned properly but if you align it then we can focus all the rays of light from the Sun to a single point and we can make that that focal point that point where it gets focused right on that slide and the slide gets eliminated all right so there's a concave mirror down over here it's a concave mirror so next time when you are using a compound microscope just dust that may touch that mirror and you can feel the concave part of it another similar place where you can see this is in your satellite dishes not satellite dishes I'm sorry these are Dish TVs these are the receivers of your dish TVs they're supposed to receive the radio signals but you can see that there is a concave reflector over there now this is not a reflector of visible light this is a reflector of radio waves now radio is a pretty similar to visible-light except that you can't see it but everything is the same almost all its properties are pretty much the same and so that these radio signals come from some radio tower which is far away and as a result we can also assume that the radio signals we get over here let's let me let me draw some radio waves the radio waves that come over here we can also pretty much assume that they are let me draw it over here it's easy to see where those radio waves you can assume they are loose okay fine so the radio waves which are coming over here we can assume that they are parallel to each other and when these parallel rays go and hit this particular reflector it reflects these radio waves to a single point as you can see all the radio waves get reflected to a single point and and that point is the principal focus and it's that that point you are going to keep your radio receiver and as a result the receiver will receive a very strong signal a very highly concentrated signal of radio waves and that that signal will not then be sent to your TV and you can watch television one more application of this same concept is in solar cookers we can use this principle to actually cook food so whatever food you want to cook you can keep it at so at the focal point of a giant mirror and then when the Sun is at the right place like you know during the noon time when the Sun is right above you we just you know orient the mirror in such a way that all the Pala rays of light are being focused on the food that you want to cook and then using the concentrated power of the Sun we can cook the food that's the idea behind solar cookers solar cookers another application it's a little bit different it is found in flashlights or headlights of a vehicle if you look at the back part over here they're reflecting that is a concave mirror back there huh why are we using a concave reflector here are we trying to focus the beam of light here as well actually no in fact over here we are trying to do something exactly the opposite so if we come back to the ray diagram one of the cool properties of these rays is that we can reverse these rays and the diagram still holds true what I mean is if you look at this ray this is the incident ray and this is the reflector right now what I'm trying to tell you is we can reverse this meaning if this is the incident ray then this would be the reflected ray let me just show you what I mean so just I'm just going to reverse all the arrow marks look at the diagram carefully just going to reverse all the arrow marks and I'm saying the diagram still holds true so let me just draw that over here and the reason that is true is because think about it even if we interchange the incident ray and the reflected ray the angle of incidence and the angle of reflection will change let me just draw that the angle of reflection before this was the angle of reflection and this was the angle of incidence but now this will be the angle of incidence and this will be the angle of reflection because this is the incident ray and is the reflected ray but it doesn't matter because they're both still equal to each other and so the rule of reflection will still work isn't it and that's one of the cool things about reflection all right you can always reverse the rays of light and it'll still hold true because the rules of reflection will still hold true now what does this mean this means I mean look at it this means that if you have a point source kept right at the focus of that mirror of that concave mirror and that source will now start giving light in all the directions if you consider the right light that falls on the mirror after reflection all the rays of light will end up becoming parallel to each other exactly the reverse before the para rays are being concentrated a single point now a ray of light emanating from a single point is becoming parallel to each other okay and so what's the use of this or whenever we want to create a highly directional beam of light well we can use a concave reflector that's exactly what's happening over here so here is the bulb over here you can see a tiny bulb over here right and that bob is placed right at the focus of this entire reflector and so if you look at the rays of light there is of light that are coming forward I just coming forward nothing will happen to them but the rays of light that are going back they hit the mirror they hit the mirror and because of this property because the rays of light are MA from the focus these rays are after hitting the mirror they get they get reflected forward and as a result this flashlight ends up producing a beam of light which is directed forward so it and that's we've seen that right flashlights produce a forward beam of light pretty directional beam of light and that's possible that's happening only because there is a reflector behind the same thing is working true this same thing also holds true for the headlights of a vehicle alright so what we did in this video is we looked at a couple of applications of concave mirrors and all the applications we looked at were based on one principle that had a rays of light when they hit the concave mirror they all get focused at a single point and of course it's reverse is also true I just let me talk about one last application before ending this video it's also a very interesting application not in light but in sound the same things even works out for sound even sound can reflect and even for reflection of sound the angle of incidence and the angle of reflection turns out to be true and so the application comes in acoustics if you're in a large hall and you want to address an audience and then we can assume that the audience is sitting somewhere somewhere let's say the audience is sitting somewhere over here and you have this large hall over here and suppose you are the speaker and you want to channel your audio you want to tell your sound in the forward direction and not side words then what you can do is you can put a you know a large concave reflector behind you now this would be a reflector of sound not light and that will ensure that any sound that goes back will get reflected forward like this and will be channeled towards your audience