Question

please tell the lens formula

Answer 1

$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$
where v=image distance
u=object distance
f=focal length.

Answer 2

v = Distance of image from optical centre of lens
u = Distance of object from optical centre of lens
f = Focal length of lens

The lens formula is applicable both in convex lenses and concave lenses.