Posted by Anonymous on Sunday, March 30, 2008 at 1:18am.
A child looks at his reflection in a spherical tree ornament 12.0 cm in diameter, and sees an image of his face that us reduced by onefourth. How far is his face from the ornament?

physics  drwls, Sunday, March 30, 2008 at 2:47am
The radius of curvature of the ornament is 6.0 cm and the focal length is half that, 3.0 cm. Because if is convex, the focal length is considered negative.
For the image position,
1/do + 1/di = 1/f = 1/3.0
Since there is a demagnification of 1/4, di = do/4
I used a minus sign because the image will be virtual, behind the reflecting surface.
1/do + 1/di = 1/do 4/do = 1/3
3/do = 1/3
do = 9 cm.