Friday 17 January 2020

reflection - Deep confusion with conventions and signs in geometric optics


This is an equation given in my book.


enter image description here


The question is why have they used a negative sign on the LHS?


Now, if you try to derive the mirror equation with simple geometry, you get 1/v +1/u =1/f . I got that equation without applying any signs there! So what is the need to apply the signs ‘here'?


If i try to derive this equation ( the one in the photo ) i get the exact answer, but with the positive sign in the LHS. WHY we use the negative sign in the derivation, and then again we use conventions while solving the questions?




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