How Do You Calculate the Focal Length of a Microscope's Eyepiece?

[kbray278]

Homework Statement

The near point of a naked eye is 25 cm. When placed at the near point and viewed by the naked eye, a tiny object would have an angular size of 5.2X10-5 rad. When viewed through a compound microscope, however, it has an angular size of -9.20X10-3 rad. (The minus sign indicates that the image produced by the microscope is inverted.) The objective of the microscope has a focal length of 2.8 cm, and the distance between the objective and the eyepiece is 16 cm. Find the focal length of the eyepiece.

Homework Equations

M=(L-Fe)(N)/FoFe

The Attempt at a Solution

Tried using the equation and solved for Fe and got the answer 15.98 but it was wrong. Any Ideas?

 

[rl.bhat]

The magnification M = angular size of the image/ angular size of the object. Find M.
Now M = 25/f(effective), where f(effective) is given by
1/f(effective) = 1/fo + 1/fe - d/fo*fe
where fo...>focal length of the objective
fe...>focal length of the eye-piece
and d...>distance between objective and eye-piece.