Question

What is difference between second principal focus and first principal focus of lens?

Answer 1

The difference between first and second principal focus of lens is that one principal focus is in front of the lens but second principal focus is back of the lens.
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Answer 2

Difference between 'second principal focus' and 'first principal focus' of lens:

The "first principal focus" of a lens is the 'position' where if an 'object' is placed, the 'image' of the object will be formed at infinity. "Second principal focus" of a lens is the 'position' on which the "image" is formed when the object is placed at infinity.

'Principal focus of a lens' is the 'position or point' on the "principal axis" where the parallel incident rays meet or refract in a lens.