Question

Derive lens formula, that is 1/v - 1/u = 1/f

Answer 1

Derivation of Lens Formula for concave lens:

Let AB represent an object placed at right angles to the principal axis at a distance greater than the focal length f of the convex lens. The image A1B1 is formed between O and F1 on the same side as the object is kept and the image is erect and virtual.

 



 

OF1 = Focal length = f

OA = Object distance = u

OA1 = Image distance = v











But from the ray diagram we see that OC = AB





From equation (1) and equation (2), we get







Dividing throughout by uvf