# Concave mirrors - where is the object

1. Nov 10, 2006

### danago

"A concave mirror of focal length 1m is used to magnify an object by a factor of 4. Where is the object in relation to the pole of the mirror?"

Heres what i did:
$$\begin{array}{c} M = \frac{v}{u} \\ \therefore 4 = \frac{v}{u} \\ \therefore v = 4u \\ \end{array}$$

$$\begin{array}{c} \frac{1}{f} = \frac{1}{u} + \frac{1}{v} \\ \therefore 1 = \frac{1}{u} + \frac{1}{{4u}} = \frac{5}{{4u}} \\ \therefore u = 1.25m \\ \end{array}$$

Where am i going wrong? According to my textbook, the answer is 0.75m.

Dan.

2. Nov 10, 2006

### Alt+F4

You are forgeting that M = -di/do

Formula = (1/do + 1/di) = (1/f)

So 4do = - di

1/do - 1/4do = 1

1/4do - 4/4do = 1

3/4do = 1

do = .75

Last edited: Nov 10, 2006
3. Nov 11, 2006

### TheloniousMONK

Right.

3/(4do) = 1 so do = .75

Last edited: Nov 11, 2006