# Image Height Question

1. Nov 17, 2013

### theintarnets

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

A man holds a double-sided spherical mirror so that he is looking directly into its convex surface, 44.8 cm from his face. The magnification of the image of his face is +0.22. What will be the image distance when he reverses the mirror (looking into its concave surface), maintaining the same distance between the mirror and his face?

2. Relevant equations

m = -di/do
1/f = 1/di + 1/do

3. The attempt at a solution

First I solved for the focal length like this:

di = - (m*do)
di = - (.22*44.8) = -9.856 cm

1/f = 1/di + 1/do
1/f = 1/-9.856 + 1/44.8
f = -12.64 cm

Then I tried to solve for the image height of the concave side:

1/di = 1/f - 1/do
1/di = 1/-12.64 - 1/40.8
di = -9.856 cm

But the answer should have been 17.6, I don't really know what to do. Please help?

2. Nov 17, 2013

### theintarnets

Oh, I think I got it! I think the signs in the formula switch to + for concave surfaces. When I do 1/di = 1/-12.64 + 1/44.8 I get -17.6. But I think my signs are wrong altogether, because it's supposed to be positive...