Q1. Which one is the correct lens formula? Here the symbols have their usual meaning.
A 1/f = 1/v + 1/u
B 1/f = 1/v - 1/u Correct
C 1/f = 1/u - 1/v
D 1/v = 1/f - 1/u
Explanation
Using the Cartesian sign convention for lenses, the standard thin lens formula is 1/f = 1/v - 1/u. The expression with a plus sign between 1/v and 1/u is not the standard lens formula. Reversing the subtraction to 1/u - 1/v changes the sign relation. The form 1/v = 1/f - 1/u is not equivalent to the standard expression; rearranging the correct formula would give 1/v = 1/f + 1/u.