# Homework Help: Thick lenses

1. Oct 15, 2006

### NutriGrainKiller

Here's the problem:

Here's what I understand:

I know what a biconcave lens looks like, and how it behaves (for the most part). Of course I know that the index of refraction is ~1.00. And since the radii are equal that makes the equation somewhat easier. The thick lens equation is:

(Sorry I am not yet familiar with LaTex, so just *imagine* that (1/R1)-(1/R2) isn't there).

What I don't understand:

Where is the image diverging from? "Parallel rays from the central axis convirge into a reflected and transmitted image"..does this just mean where the rays convirge?

Any guidance would be appreciated. Thanks as always!

Last edited by a moderator: Apr 22, 2017
2. Oct 15, 2006

### OlderDan

So, can we reduce the equation to

1/f = d/2R^2

For parallel incoming rays the "object" is at infinity and the reflected image is a point. For spherical reflectors this point is half a radius from the surface. I'm not sure I follow the sign convention for the formula, and it's not immediately clear to me how the "transmitted image" will appear. A biconcave lens would form a virtual image, and with thin lenses that would mean a negative focal length. I know that not all sign conventions are the same, so maybe all is OK. I can see how the "transmitted image" could mean the virtual image of the diverging lens. It's not obvious that d has no effect on the position of the virtual image, but maybe that is the point of the problem

Last edited by a moderator: Apr 22, 2017