# Homework Help: Spherical mirror;find focal length from given data?

1. Jan 28, 2012

### vkash

1. The problem statement, all variables and given/known data

When an object is at distance u1 and u2 from pole of concave mirror, images of same size are formed. Find the focal length of the mirror.

2. Relevant equations

1/v+1/u=1/f

3. The attempt at a solution

Can u please ray diagram for this question. I think their is only one point from which image of size of object form(that's 2f). But it is giving two points. So how to do it..

2. Jan 29, 2012

### vela

Staff Emeritus
I think you're misreading the question. It's not saying the image is the same size as the object. It's saying the image formed when the object is at u1 is the same size as the image formed when the object is at u2.

3. Apr 10, 2012

### vkash

OK!!
acha ji.()

but is it possible that two points have same size of image(for a fixed object). I think it will decrease as we take object from pole of the lens to infinity...

please tell me how to proceed..

4. Apr 10, 2012

### vela

Staff Emeritus
Think virtual and real images.