Finding object distance with converging lenses.

[ilovedeathcab]

A converging lens with f=33.7 cm yields an image with magnification M=−2. Find the object distance. Give your answer ...

* in terms of centimeters. Do not include units in your answer. They will be assumed to be cm.
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I'm not really sure how to approach this problem. I keep calculating it and getting the wrong answer. I started with the 1/f =1/do + 1/di equation, and know that M = -di/do, but from there my work is probably wrong

-2=-di/do : -2do = -di ----> 2do=di

1/f=1/do + 1/2do

1/f =1/3do

f= 3do
1/3f=do
1/3 * 33.7cm =do
do= 11.23cm =WRONG.
?

 

[nickjer]

Ok, your mistake is going from the 2nd line to the 3rd line.

You didn't add the fractions properly. What is 1+1/2?

 

[ilovedeathcab]

I'm still confused... 1/f= 3/2do? i don't know where to go from there

 

[nickjer]

Solve for 'do' since you know what 'f' is.

 

[ilovedeathcab]

thanks! i got the right answer this time...