In the last video, I showed that a bunch of triangles
are similar to each other to come up
with the relationship between the focal length,
the distance of an object from the convex lens,
and the distance of the image of an object from that convex
lens.
And I realized that there was one extra low-hanging fruit
based on all of the geometry that I had done,
another relationship that might come in useful.
And that's the relationship between the size
of the object-- or maybe since it's an arrow here,
we can call it the height of the object,
and the height of the image.
And we really set up everything already.
We already figured out that this triangle over here
is similar to this triangle down here.
And we figured out in the last video
that this triangle over here is similar to this triangle
over here.
And since these two are similar, we
could say that A is to B-- we did this over here.
I'll rewrite it.
A is to B as-- and both of those are the sides
opposite the right angles of these two similar triangles.
So that's going to be the same thing as the ratio of the sides
opposite this yellow angle right over here.
So in this triangle over here, since we started with A first,
it's this height right over here.
Now what is this height right over here?
This is the height of the object.
So this is the height of the object
is to-- Now what is this opposite side
of this yellow angle right over here?
Well, this is the height of the image.
Or we know from the last video the distance
of the object to the distance of the image
is the same thing as A to B.
So this is going to be the same thing as this.
So the ratio of the distances is also
the same thing as the ratio of their heights.
So let me write it this way.
So the ratio of the distance from the object to the lens,
to the distance from the image to the lens,
is the same as the ratio of the height
of the object to the height of an image,
or to the image of that object.
So I just wanted to do that little low-hanging fruit there,
since we set up all of the mechanics already.
Anyway, hopefully you found that useful.