# Focal length of a mirror

1. Jul 16, 2013

### aChordate

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

The image of an object seen in a mirror is twice its actual size. The orientation of the
image is the same as that of the object. If the object distance do is 1.7 m, what is the focal
length of the mirror?

2. Relevant equations

1/do+1/di=1/f

m=-di/do

3. The attempt at a solution

(1/1.7)+1/di=1/f

di=m*do=2*1.7m=3.4

(1/1.7)+(1/3.4)=1/f
f=1.13m

2. Jul 17, 2013

### ehild

di = -mdo = -3.4

ehild

3. Jul 17, 2013

### PeterO

For an enlarged image, you must have a concave mirrror. For the image to be upright the object must be inside the focus, so the focal length is going to be bigger that 1.7 m.
A simple sketch of a ray trace will give you this answer in 5 seconds.

If you have drawn enough ray traces before, you will be able to draw this one in your mind rather than committing pencil to paper.
If you haven't drawn enough ray traces to do that, then you better get busy drawing a few more ready for next time.

Last edited: Jul 17, 2013