State the sign convention to measure the distances for a lens.

We follow the cartesian sign convention to measure the distance in a lens according to which:

1. The optical centre of the lens is chosen as the origin of the coordinate system.
2. The object is considered to be placed on the principle axis to the left of the lens.
3. All the distances are measured along the principle axis from the optical centre of the lens. The distance of the object from the lens is denoted by u, the distance of the image by v and the distance of the second focus by f.
4. The distances measured in the direction of the incident ray are taken positive, while the distances opposite to the direction of incident ray are taken negative.
5. The length above the principle axis is taken positive, while the length below the principle axis is taken negative.
6. By sign convention, the focal length of the convex lens is positive and that of the concave lens is negative.
7. The distance of object (u) infront of lens is always negative. The distance of image (v) is positive if it is real and formed behind the lens, while it is negative if the image is virtual and formed in front of the lens.