Focal length

1. Apr 30, 2007

wakejosh

1. The problem statement, all variables and given/known data
An object and a screen are separated by 20 cm. A convex lens is placed between them, 5 cm from the object. In this position it causes a sharp image of the object to form on the screen. What is the focal length of the lens?

3. The attempt at a solution
wouldn't it just be 5 cm since it is producing a sharp image on the screen? the distance from the object to the lense?

2. Apr 30, 2007

wakejosh

looking at this more I think i need an equation. I dont have a book so can someone show me the equation im looking for?

3. Apr 30, 2007

hage567

You need the thin lens equation:

$$\frac{1}{f} = \frac{1}{p} + \frac{1}{q}$$

where f is the focal length, p is the object distance, and q is the image distance.