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We've talked a lot about convex lenses,

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so I thought I would do a quick video on concave lenses,

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although there aren't as many combinations

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of what a concave lens can do.

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So just remember,

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concave has the word 'cave' in it

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so I always imagine that it's kind of caving inwards

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caving inwards

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It's a little bit of an exaggerated drawing

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but you get the general idea

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this is a concave lens right over here

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and let me draw its principle axis

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so this is its princinpal axis

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shouldn't curve at the beginning

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just redraw it, so that's its principle axis

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and let me draw the two focal points

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let me draw one over here on the left side

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and then an equal distance focal distance on the right side

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and I'm assuming that the two surfaces are both concave

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and they are symmetric over here

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Just an assumption I'm making

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Now lets think about what's gonna happen

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if I put an object some place

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on the left side of this concave lens

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So I can put it, let me just stick it anywhere

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So let me just stick it right over there

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So, if I do and like always,

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we will do our two rays but with a concave lens,

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I'm only gonna do two rays

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one that is parallel to the principle axis

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and then I'm gonna do one that does not get diffracted

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so it goes right through the center of the lens

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and let's think about what happens.

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So, if we go parallel, to the principal axis,

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this will get diffracted away, since it's coming in parallel

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and it will get diffracted outwards

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so that it looks like it's coming from this focus.

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So it will get diffracted in a way

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so it looks like its coming from that focus

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So its getting diverged outwards

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Let me draw another point or another ray, I should say

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I will do that in yellow

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and this will just gonna straight

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through the centre of the lens and not get diffracted

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So its just gonna go straight through the lens

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and not get diffracted

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So what will the image be here?

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Well clearly, these two rays don't converge

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so we won't have a real image

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but they do both look like they are diverging from someplace

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and they look like they are diverging from right over there

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so look to an observer out here

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if their eyeball is right over there

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I could draw their nose, just make it clear where the eye is

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If they look at from this side,

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they are going to see a virtual image of the image right over here

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Just like that

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So you're actually gonna see is a virtual image of the object

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that is not inverted and it's gonna be smaller

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its gonna look like it's closer to the lens than it really is

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Anyway, that's all it really is to know,

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obviously there's a lot you can know about anything.

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but that's the general gist about the concave lenses