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Principal focus of a mirror (lens) is the point on the principal axis of the mirror (lens) at which point the parallel rays (parallel with the axis) which are incident on the mirror (lens) converge.

so for convex lens and convex mirror, the principal focus is on the other side of the device.  for a concave mirror and concave lens, it is on the same side of the device.
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Pricipal focus is the point on principal axis where lights converges. 
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