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Old Jul 6th 2009, 03:45 AM   #1
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Finding focal length

Used as a magnifying glass, the image of an object 4cm from a converging lens is five times the object length. What is the focal length of the lens?

I used m=v/u=5
v=4*5=20cm
the i used 1/v+1/u=1/f
1/20+1/4=1/f
f=3.33cm
the problem is here, the image is supposed to be same way up and virtual, that means that the focal length is greater than the object distance u. the answer is supposed to be 5cm.
where have i gone wrong?
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Old Jul 6th 2009, 08:34 AM   #2
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Originally Posted by arze View Post
the i used 1/v+1/u=1/f

Formula for lens is

$\displaystyle \frac{1}{v}+\frac{1}{u}=\frac{1}{f}$

you have used mirror formula instead of lens formula.
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Old Jul 6th 2009, 09:34 PM   #3
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The value of a virtual image is negative.

$\displaystyle
\frac{1}{u}+\frac{1}{v}=\frac{1}{f}
$
$\displaystyle
\frac{1}{4}+\frac{1}{-20}=\frac{1}{f}
$
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