Simple Microscope Problem that I have been STUCK on

[BuBbLeS01]

Simple Microscope Problem that I have been STUCK on!

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?

Homework Equations

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??

 

[BuBbLeS01]

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]

I don't understand this

M = s'-f/f

Shouldn't it be just M = -s'/s?

 

[BuBbLeS01]

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]

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.

 

[BuBbLeS01]

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]

1/ms + 1/s = 1/f

This term should be negative since s' = -ms

 

[BuBbLeS01]

Thank you soooooooooooooooooo much for your help!