# Focal length of a convex mirror

1. Jul 16, 2013

### aChordate

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

An object is 12.0 cm in front of a convex mirror. When the convex mirror is replaced
with a flat mirror, the image distance is 5.0 cm farther away from the mirror. What is the
focal length of the convex mirror?

2. Relevant equations

f=-1/2R

1/do+1/di=1/f

3. The attempt at a solution

(1/0.12m)+1/di=1/f
?

12+5cm=17cm=0.17m

(1/0.17m)+1/di=1/f
?

2. Jul 17, 2013

### ehild

How is the image distance related to the object distance in case of a flat mirror?

ehild

3. Jul 17, 2013

### aChordate

They would be equidistant to the mirror?

4. Jul 17, 2013

### ehild

Yes. So the mirror image in case of flat mirror is farther away by 5 cm than the image produced by the convex mirror. If the object distance is 12 cm and the image distance is shorter by 5 cm, what can be di?

ehild

5. Jul 17, 2013

### aChordate

di=12cm for the flat mirror
di=7cm for the convex mirror?

6. Jul 17, 2013

### aChordate

Using the above di, I got f=0.12m.

7. Jul 17, 2013

### Basic_Physics

The image is behind the mirror.

8. Jul 17, 2013

### ehild

The image is at 7 cm distance from the mirror, but the mirror is convex, is the image virtual or real?
What is the sign of di ?

ehild

9. Jul 17, 2013

### PeterO

Have you solved the problem? A quick ray Trace will do it for you.

10. Jul 18, 2013

### Basic_Physics

The problem statement tells us that the image is behind the mirror.