Equivalent Focal Length Of Lenses In Contact — the NEET Physics formula with its derivation, variables, validity constraints and worked solver.
Equivalent Focal Length of Lenses in Contact Two or more thin lenses placed in contact coaxially in a medium. Assume a point object placed in front of the first lens The image formed by the first lens acts as a virtual object for the second lens Apply the thin lens formula 1/v - 1/u = 1/f to each lens sequentially Sum the equations to eliminate the intermediate image distance Lenses must be thin to ignore thickness Lenses must be in physical contact Lenses must share the same principal axis Surrounding medium must be consistent Forgetting to use sign conventions for concave lenses (negative focal length) Applying the formula when there is a finite distance 'd' between lenses Assuming the formula holds for thick lenses where thickness is comparable to focal length