# Deep confusion with conventions and signs in geometric optics

This is an equation given in my book.

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?

The formula you have given in the question is used for finding image position due to refraction at spherical surfaces.

The mirrors work on the principle of reflection and there is no way you could adjust that formula to work with mirrors. In fact, the refractive index(s) in the formula make no sense for a mirror.

Secondly, most of the proofs in Optics are geometrical proofs where we use the distance between one point and another. After deriving the equation, we apply the sign convention so that anyone using the formula can directly substitute values as per convention.

For example, in our geometric derivations, we use $|u|$ as the distance of the object from the mirror/lens. If the object was behind the mirror, u is negative, so substituting $u$ with the sign in our formula would yield a wrong answer. Hence, we apply the sign convention to the formula once so that in future, the values can be substituted with signs. So if $u = -20$, the $-u$ will change it to 20 which is what the geometric formula expects (it needs distances).