# Homework Help: Need help understanding wording of simple geometric optics problem

1. Oct 15, 2006

### NutriGrainKiller

I understand the material this problem is based on very well, but I feel very sad when I read this problem because it makes very little sense to me. I'm hoping it's just me that's confused by it and not everyone else - because in that case I should be able to get help here.

Here is the problem:

grammar, spelling etc.. is verbatim. Again I don't so much need help doing this problem as I do understanding it. Any input at all is appreciated. Thanks!

2. Oct 15, 2006

### OlderDan

Draw a positive thin lens (convex lens, converging lens) wih an axis through the center in the usual way. Mark the focal points on both sides of the lens, and points at two focal lengths. On the left side of the lens place a point source on the axis. Use the lens equation to locate the image of this object. Light coming through the lens would normally focus at this image point. You are going to use a mirror to reflect the light back through the lens in such a way as to focus the light at its point of origin. You know the mirror is spherical, but it does not tell you the radius of the sphere.