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: The [[Focal Length|focal length]] of a '''convex lens''' is the distance between the [[Focal Point|focal point]] and the centre of the [[lens]].
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|[[File:FocalLength.png|center|500px]]
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| style="height:20px; width:200px; text-align:center;" |A [[diagram]] showing how to find the [[Focal Length|focal length]] of a '''convex [[lens]]'''.
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: To find the [[Focal Length]] of a '''convex [[lens]]''':
*[[Focus]] the [[image]] of a "distant object" onto a [[screen]].
*[[Measure]] the [[distance]] between the [[lens]] and the [[screen]].
: The [[Image Distance|image distance]] (the distance between the [[focus]]sed [[image]] and the [[lens]]) is the same as the [[Focal Length|focal length]] for a "distant object".
: If the [[Object Distance|object distance]] (the distance between an [[object]] and a [[lens]]) is too small, then the [[Focal Length|focal length]] is not the same as the [[Image Distance|image distance]].