# Lens equation

by Sam Morse
Tags: equation, lens
 P: 15 I googled for lens formula's proof on the internet and found one here http://www.tutorvista.com/content/ph...ns-formula.php The problem is that during the course of derivation, it is assumed that image distance is -u following the sign convention. But I don't see why we have to use to sign convention once again while solving problems in ray optics...
P: 694
 Quote by Sam Morse I googled for lens formula's proof on the internet and found one here http://www.tutorvista.com/content/ph...ns-formula.php The problem is that during the course of derivation, it is assumed that image distance is -u following the sign convention. But I don't see why we have to use to sign convention once again while solving problems in ray optics...
We have chosen the sign convention to maintain consistency while solving the problems.

Direction has been considered....
 P: 15 I understand why we use sign convention ... but what's the need of using the sign convention once again ? We have already used the sign convention while deriving the formula for a lens.
P: 694

## Lens equation

 Quote by Sam Morse I understand why we use sign convention ... but what's the need of using the sign convention once again ? We have already used the sign convention while deriving the formula for a lens.
Perhaps, I am of the opinion that while deriving lens formula we use the concept of similar triangles and that formula : 1/f=1/v-1/u is sign independent for now, unless while solving the problems we consider the sign convention.

http://www.tutorvista.com/content/ph...ns-formula.php
PF Patron