Thin Lens Formula — the NEET Physics formula with its derivation, variables, validity constraints and worked solver.
Thin Lens Formula Applies to thin lenses where the thickness is negligible compared to the radii of curvature, and for paraxial rays. Assume a thin lens with optical center O Draw a ray diagram for a point object on the principal axis Use similarity of triangles (e.g., triangle ABO and triangle A'B'O) Express the ratios of sides in terms of u, v, and f Apply the Cartesian sign convention to arrive at the final subtraction form Lens must be thin Rays must be paraxial (close to the principal axis) Medium on both sides of the lens must be the same Standard Cartesian sign convention must be followed Confusing with the Mirror Formula where the sign is positive (1/v + 1/u = 1/f) Forgetting to apply sign conventions (u is usually negative for real objects) Applying it to thick lenses without correcting for thickness