Question

How to find focal length of convex lens by u v method?

Answer 1

now by uv method we know that
$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$
now for convex lens
u=negative
v=positive
f=positive by this we can find focal length for eg.
let v=+30cm,u=-15cm

now we know that
1\v-1\u=1\f
1\30 + 1\-15=1\f
15+30\450=1\f
30+15\450= 1\f
therefore f= +10

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 .