# Simple Microscope Problem that I have been STUCK on

Simple Microscope Problem that I have been STUCK on!!!

1. Homework Statement
How far from a lens of focal length 50 mm must the object be placed if it is to form a virtual image magnified in size by a factor of 3?

2. Homework Equations

3. The Attempt at a Solution
I thought I could us the equation...
M = s'-f/f to get s' and then use...
1/f = 1/s + 1/s' to get the object distance

but I am not coming up with the right answer??

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Can anyone help me on this? I still can't get the answer? I am suppose to get 33 mm and I m getting 40 mm

hage567
Homework Helper
I don't understand this
M = s'-f/f
Shouldn't it be just M = -s'/s?

I don't know its in my book...
but I don't have s' or s??
I know F = 50 mm so...
50 = (1/s + 1/s')^-1
but I am still not getting it?

hage567
Homework Helper
You are trying to find s, right? You were given M so you can solve for s' using the magnification equation and then substitute that into the lens equation which will let you solve for s.

Ok sooo...
-s' = ms
s' = -ms
1/ms + 1/s = 1/f
1/s * (1/m + 1/s) = 1/f
1/s = [(1/f) / (1/m + 1)]^-1 = 66.67??? what am I doing wrong??

hage567
Homework Helper
1/ms + 1/s = 1/f
This term should be negative since s' = -ms

Thank you soooooooooooooooooo much for your help!!!!!!!