How Does Lens Focal Length Affect Image Size?

[thurrucane]

Homework Statement

An object is located 26.0cm from a certain lens. The lens forms a real image that is twice as high as the object.

(i) What is the focal length of this lens?

(ii) Now replace the lens used in Part 1 with another lens. The new lens is a diverging lens whose focal points are at the same distance from the lens as the focal points of the first lens. If the object is 5.00cm high, what is the height of the image formed by the new lens? The object is still located 26.0cm from the lens.

Homework Equations

Since the image is real and 2X as large, the magnification m = -2. The formula
m = -i/o
1/o + 1/i = 1/f

The Attempt at a Solution

After attempting part 1 i got 5.77cm which is incorrect.

 

[hermy]

You got to take care of the sign convention.