Question

Formula for focal length for concave lens

Answer 1

Concave lens:

  Lens Formula for focal length/image:   1/f = 1/v - 1/u

    Since concave lens is diverging lens, virtual and erect diminished images are formed.  As per the convention of signs :
 
    f = negative, v = negative and u is negative as the focus, object and image are all on the same side.

Lens maker's formula for a concave lens :
 
     1/f = (μ₂/μ₁ - 1) [1/R1 - 1/R2]

     μ₁ and μ₂ are refractive indices for the air and glass respectively.

     R1 and R2 are radii of curvatures of object side and opposite side      spherical surfaces of the lens.   R1 is negative and R2 is positive.  f is negative.

Answer 2

concave lens formula :-

1/f= 1/v-1/u