Define principal focus of a lens ?

Principal Focus is a point on the principal axis where the beam of light travelling parallel to the principal axis after refraction through the lens actually meet or appears to meet...

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please specify the type of lens???

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When a narrow parallel beam of light is incident on a lens parallel to its principal axis the beam after refraction through the lens, converges to or appear to diverge from a fixed point, on the principal axis. This point is called the principal focus of the lens.

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