Question

What is lens formula? Give its sign conventions.

Answer 1

Following is the Lens Formula:

1/v - 1/u = 1/F, where,

V=Image distance
U=Object Distance
F=Focal Length.

All measurements are made from the optical center of the lens. Therefore, an easy way of remembering it is that all measurements made IN THE DIRECTION OF THE INCIDENT RAY are positive.

For example if the object is present to the left of a convex lens, the focal length will be positive as it is measured for the refracting surface towards the right. And since the direction of measurement of the object distance is opposite of the incident ray, it will be negative. And the sign of the image distance depends on what side of the refracting surface it is formed.

Answer 2

                         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 .