Question

What is lens formula?what are the conventions and assumptions in lens formula?

Answer 1

Lens formula gives relation between object distance, image distance and focal length of lens
1/f=1/v-1/u
New Cartesian Sign conventions:
1. All distances are measured from the optical centre of the lens.
2. The distances measured in the direction of incident light are taken as positive and those measured in opposite direction to the incident light are taken as negative.
3. The focal length of a convex lens is positive and concave is negative.
The following assumptions are made in the derivation of the formula:
1. The lens is thin
2. The lens has small aperture
3. The point lie on to the principal axis and placed perpendicular.
4. The incident ray makes small angles with the lens surface or the principal axis.

Answer 2

Lens formula: 1/f=1/v-1/u

Sign conventions in convex lens:
focal length is always positive
the distance of the object is always negative
If the image formed is real and inverted than the image distance is positive
If the image is virtual and erect than the image distance is negative

sign conventions for Concave lens:
focal length is always negative
The distance of an object is always negative
The distance of an image is always negative