# Light/mirror questions.

1. Nov 21, 2012

### skg94

1. The problem statement, all variables and given/known data
1. A student wishes to place an object in front of a concave mirror in order to produce an image one-half the object's size. If the focal length of the mirror is 5.0cm, how far from the mirror should the object be placed? ANS : 15 cm.

2.An object is placed 2.0cm beyond the centre of curvatur of a concave mirror. If the magnification of this object is 0.30 what is the radius of curvature of this mirror. ANS: 1.7cm

2. Relevant equations

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

3. The attempt at a solution

1. I believe these are questions where you have to combine two formulas, i dont know how to go about it, im not asking for full work, if oyu decide to show full steps, im trying to figure it out and i cant seem to do it, can someone start me off on the right track? Or if im overthinking it put me on the right track.

1) 1/2ho/ho= f-do/do
1/2do=-f-do
2do=-2f
which would be -5, which isnt right.

2. Nov 21, 2012

### ehild

What do you mean with equation (1)? ho/ho=1 and do/do=1, so eq(1) is equivalent to 1/2=f-1 which is obviously wrong.

ehild

3. Nov 22, 2012

### skg94

well hi is half of ho which makes it 1/2ho, so i replaced hi with 1/2ho
and i replaced di with f-do from

f=do+di

4. Nov 22, 2012

### haruspex

No, 1/f = 1/do+1/di. That's quite different.