Solving for Magnification in a Microscope

[Linus Pauling]

1. Your task in physics laboratory is to make a microscope from two lenses. One lens has a focal length of 2.8 cm, the other 1.1 cm. You plan to use the more powerful lens as the objective, and you want the eyepiece to be 15 cm from the objective.

What is the magnification of your microscope?



2. Thin lens equation
M_obj = -s'/s
M_eye = ?



3. In part one I solved for s, and s = 1.2cm. The more powerful lens is the one with f = 1.1, and s' = 13.2. Thus, the magnification of the objective lens should be -s'/s = -13.2/1.2 = -11. Is that correct? Also, what do I sue for the magnification of the eye? I know that I have to mutiply those two values to obtain my final answer.

 

[Linus Pauling]

This one's really killing me guys. I think I've got all my sign conventions right, and none of the incorrect answers I've tried have said, "check your signs." I've tried the s'=15cm for the image distance (which I used to calculate s in part one, which was correct...) but that didn't work, i.e. -(15/1.1)(25/2.8). I also tried using s'=15-2.8=12.2, putting the image from the objective projected onto the focal point of the eyepiece.

wtf...

 

[Linus Pauling]

The answer is in the back of the book, but with different numerical values. I worked out how to get the correct answer for the book numbers, then applied the exact same logic to the question with the values I am asking about... and it says it's wrong...