We looked at the power and the focal length of a single thick lens here.

We can also apply the same types of techniques to a two lens system.

Right now I'm going to look at a positive lens.

And find the focal length that is positive lens.

It does have some thickness.

That's why I'll need to trace my rays back along here.

My parallel rays coming in and see where they overlap.

Conceivably somewhere inside this lens,

and measure my focal length.

And I have a negative lens.

I can do the same thing, I can take these rays and follow them all the way back

here to find the front focus for my virtual images because I have diverging rays.

And we can see where those rays overlap with these parallel rays.

This is a fairly thin lens so maybe right here in

the center where I can measure my focal length of this lens.

Now what happens when I put those two lenses together?

These are designed to fit pretty closely together.

And if I put these two lenses together and I send in parallel rays,

we get almost parallel rays out.

So what does that tell me about the two focal lengths of the lenses?

Are they two powers these two lenses?

Another total power of the system,

so I have rays coming in and going at the same angle [inaudible] easier.

So the power of one of these lenses needs to

be equal and opposite to the other which gives

me that these focal lengths is effective focal length

also need to be pretty close to equal and opposite.

They're not perfect rays there is converging here a little bit.

Now as I move this negative lens further out,

if I increased the distance between these two lenses,

what's going to happen to the rays?

What are those rays going to do as they travel through the lenses?

What does it mean about the power of the system if we

take these two lenses and treat them as a single lens?

What is the power of that new two lens system as I increase

the distance between the two? Let's go ahead and do it.

So as I go up further and further,

it looks like the focus is coming [inaudible] at the focal length,

effective focal length, of this two lens system.

The distance from the principal planes that's where we draw

these back and see where they overlap with the parallel rays coming in,

it should be in here somewhere.

This effective focal length is now shorter.

I have a shorter focal length

and so the power of my system is greater.

And as I increase d the power in that focal length,

the focal length decreases and the power increases.

And as I decrease d to almost zero my power

goes to approximately zero as well because I have matched focal length here.