# Converging Lens

## Homework Statement

The distance between an object and its image is fixed at 40.0 cm. A converging lens of focal length f = 4.2 cm forms a sharp image for two positions of the lens. What is the distance a between these two positions?

1/p + 1/q = 1/f

## The Attempt at a Solution

I am very confused with this problem and really have no idea where to begin. If anyone can provide some assistance, that would be great. Thanks.

## Answers and Replies

hage567
Homework Helper
You have the lens equation. You know f. You also know that p+q=40. So what you have is two equations and two unknowns. Try solving for p (or q) and see what you come up with.