The lens formula looks almost like the mirror formula, but with a minus sign — small difference, easy to mix up in exams.
Distances are measured from the optical centre O: object distance u is negative, image distance v is positive here — a real image case.
Lenses use the same sign convention style as mirrors, but measured from the optical centre instead of the pole. Distances are measured from O, with the same left-negative/right-positive rule.
The lens formula relates object distance (u), image distance (v), and focal length (f): 1/v − 1/u = 1/f. Notice the minus sign — this is the single most common thing students mix up with the mirror formula, which uses a plus.
Magnification for a lens is m = h'/h = v/u. A positive m means the image is virtual and erect; a negative m means it's real and inverted. By convention, a convex lens has a positive focal length, and a concave lens has a negative focal length.
Key exam points
Watch it explained
Lens Formula and Magnification — Light Chapter, Class 10 Physics · Letstute CBSE