Physics Help Forum Finding focal length

 Light and Optics Light and Optics Physics Help Forum

 Jul 6th 2009, 02:45 AM #1 Senior Member   Join Date: Feb 2009 Location: Singapore Posts: 206 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?
Jul 6th 2009, 07:34 AM   #2
Junior Member

Join Date: Jun 2009
Posts: 4
 Originally Posted by arze 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.

 Jul 6th 2009, 08:34 PM #3 Senior Member   Join Date: Apr 2008 Location: HK Posts: 886 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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