All distances are measured from the optical centre of the lens.
The distances measured in the direction of the incident light are taken as positive and the distances measured in the direction opposite to the direction of incident light are taken as negative.
All measurements made above the principal axis are taken as positive and the measurements made below the principal axis are taken as negative, i.e., object height is always taken as positive and the image height is positive only for virtual image.
Formation of Image by a Convex Lens
A ray of light passing through the optical centre of the lens travels straight without suffering any deviation. This holds good only in the case of a thin lens.
An incident ray parallel to the principal axis after refraction passes through the focus.
An incident ray passing through the focus of a lens emerges parallel to the principal axis after refraction.
