Question

what are the sign convention of mirror and lens?

Answer 1

Lens Formula gives the relationship between object distance (u), image image-distance (v) and the focal length (f ) and is expressed as
1f=1v−1u
This formula is valid in all situations for any spherical lens.The magnification produced by a lens is defined as the ratio of the height of the image and the height of the object.
Magnification produced by a lens is also related to the object-distance u, and the image-distance v and is given by
m=vuThe power of a lens is defined as the reciprocal of its focal length. It is represented by the letter P. The power P of a lens of focal length f is given by
P=1fPower of a convex lens is positive and that of a concave lens is negative.The SI unit of power of a lens is ‘dioptre’. It is denoted by the letter D.1 dioptre is the power of a lens whose focal length is 1 meter so, 1D=1m–1.

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 .