Question

Why do we use sign convention again in lens stackexchange?

Answer 1

because a lens is the combination of two spherical mirrors.

Answer 2

SIGN CONVENTION

                         Note :

• f = focal length

• v = distance of image .

• u = distance of object .

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 .

WHY WE USE ?

• For calculations

• For finding the direction of image.

• For identifying types of lens

• For finding the direction of object .