Posted by **Daniel** on Saturday, March 17, 2012 at 11:37pm.

A convex mirror in an amusement park has a radius of curvature of 3.00m. A man stands in front of the mirror so that his image is half as tall as his actual height. At what distance must the man focus his eyes in order to see his image?

I calculated the focal length using f=(-.5)(3.00m)

then I tried to find the image distance(di) but I get stuck.

What am I supposed to be doing here?

- physics -
**drwls**, Sunday, March 18, 2012 at 1:36am
You have correctly determined that f = -1.5 m for this mirror. From the magnification of 1/2, and the fact that it is a virtual image, you also know that

di/do = -1/2.

Therefore 1/di + 1/do = 1/di - 1/2di =

1/2di = 1/f = -2/3

Therefore di = -3/2 m and do = 3 m

Since the observer is the "object" and his image is on the other side of the mirror, he must focus his eyes 4.5 meters away.

Check my thinking. I could have made a mistake somewhere

- physics -
**scott**, Thursday, April 11, 2013 at 1:35pm
you kind of just droped 1/di there. where did it go?

I just don't get how you got to:

1/di - 1/2di

and then you went from

1/di - 1/2di = 1/2di.....you just droped 1/di. That doesnt make any since.

- physics -
**sammy**, Monday, March 21, 2016 at 9:47pm
1/di could also be written 2/2di then if you take away 1/2di you are left with 1/2di. It's like saying 1 minus 1/2 equals 1/2

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