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Concave Lenses. Created by Sal Khan.
Video transcript
We've talked a lot about convex lenses, so I thought I would do a quick video on concave lenses, although there aren't as many combinations of what a concave lens can do. So just remember, concave has the word 'cave' in it so I always imagine that it's kind of caving inwards caving inwards It's a little bit of an exaggerated drawing but you get the general idea this is a concave lens right over here and let me draw its principle axis so this is its princinpal axis shouldn't curve at the beginning just redraw it, so that's its principle axis and let me draw the two focal points let me draw one over here on the left side and then an equal distance focal distance on the right side and I'm assuming that the two surfaces are both concave and they are symmetric over here Just an assumption I'm making Now lets think about what's gonna happen if I put an object some place on the left side of this concave lens So I can put it, let me just stick it anywhere So let me just stick it right over there So, if I do and like always, we will do our two rays but with a concave lens, I'm only gonna do two rays one that is parallel to the principle axis and then I'm gonna do one that does not get diffracted so it goes right through the center of the lens and let's think about what happens. So, if we go parallel, to the principal axis, this will get diffracted away, since it's coming in parallel and it will get diffracted outwards so that it looks like it's coming from this focus. So it will get diffracted in a way so it looks like its coming from that focus So its getting diverged outwards Let me draw another point or another ray, I should say I will do that in yellow and this will just gonna straight through the centre of the lens and not get diffracted So its just gonna go straight through the lens and not get diffracted So what will the image be here? Well clearly, these two rays don't converge so we won't have a real image but they do both look like they are diverging from someplace and they look like they are diverging from right over there so look to an observer out here if their eyeball is right over there I could draw their nose, just make it clear where the eye is If they look at from this side, they are going to see a virtual image of the image right over here Just like that So you're actually gonna see is a virtual image of the object that is not inverted and it's gonna be smaller its gonna look like it's closer to the lens than it really is Anyway, that's all it really is to know, obviously there's a lot you can know about anything. but that's the general gist about the concave lenses