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