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generalclip Jan 24th 2009 08:49 PM

Focal Length...
 
Got some work from my dad, but one question kinda stumped me.

A short sighted person can view an object clearly only when it is between 11 cm and 19 cm from her eyes. Calculate:
1. The Focal Length of the specatacles required to give her a near point of 25 cm.

I don't know what equation to use here. I usually see questions about image and object distances and object distances using the 1/u+ 1/v= 1/f equation, I've never seen one like this. Can anyone help me out here?


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