# Homework Help: Convex mirrors - Sign convention trouble

1. Feb 23, 2009

### Elbobo

1. The problem statement, all variables and given/known data
I solved the problem, but upon further observation, I discovered that what I did didn't make sense.

(these are rounded numbers, which shouldn't make a difference in my question)
do = 13 cm (do is what I solved for in the problem, and according to the online system it was correct)
f = 24 cm
di = 33 cm

(the man is facing a convex mirror with his image behind the mirror)

2. Relevant equations
1/f = 1/do + 1/di

3. The attempt at a solution
Since this is a convex mirror, f and di should be negative. Yet, it doesn't make sense with those conventions.

1/f = 1/do + 1/di
1/do = 1/f - 1/di
1/do = (1/-24) - (1/-33)
do = -88 cm,

which means the object is behind the mirror and my methodology was inconsistent.

Where is the issue? Is it just an inconsistency with sign conventions by them?

2. Feb 23, 2009

### rl.bhat

In the convex mirror, the image cannot be formed beyond the focal length.

3. Feb 23, 2009

### Elbobo

OH! I finally see what I did wrong. I wasn't thinking and at the moment thought that the hubcap is normally viewed as convex. Now I see...

Last edited: Feb 23, 2009