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2014-11-01T19:01:08+05:30
The length between the pole and focus point
 PF = f
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2014-11-01T19:01:10+05:30
The focal length of an optical system is a measure of how strongly the system converges or diverges light.For an optical system in air it is the distance in which over which intially colimated rays are brought to focus.

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The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus. A system with a shorter focal length has greater optical power than one with a long focal length; that is, it bends the rays more strongly, bringing them to a focus in a shorter distance.
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