Lens equation

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...

sankalpmittal

We have chosen the sign convention to maintain consistency while solving the problems.

Direction has been considered....

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.

sankalpmittal

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

sophiecentaur

If you don't use a sign convention the how do you work out whether the image will end up in front of or behind the lens? Also, how do you distinguish between a concave or convex lens? (The answers to those questions are not allowed to include "you specify in words, which side and which type" - the calculation must involve just Maths.)