Question

what are the sign conventions for spherical mirror and lens .

Answer 1

Object is always placed to the left of mirrorAll distances are measured from the pole of the mirror.Distances measured in the direction of the incident ray are positive and the distances measured in the direction opposite to that of the incident rays are negative.Distances measured along y-axis above the principal axis are positive and that measured along y-axis below the principal axis are negative........

Answer 2

Lens formula :

1 / f = 1 / v - 1 / u

Concave lens :

f = negative

u = negative

v =  negative

Convex lens

f = positive

u = negative

v = positive for real image ,

v = negative for virtual image .

The optical centre is chosen as the origin of the coordinate system.

With respect to optical centre :

The part which are above are positive, below negative.

The part which are right are positive while left is negative.

This is just like the normal x-axis and y-axis in the co-ordinate system .