The situation which has to be considered is the lens makers formula accounting for objects that are present in different media. The equation is as follows,

1/f=[n₁/n₂ − 1] × [(R₁ − R₂)/R₁ × R₂]

Where,

R₁ is the radius of curvature of sphere 1

R₂ is the radius of curvature of sphere 2

n₁ is the refractive index of the lens in consideration

n₂ is the refractive index of the medium outside

So,now according to question if we put n₁=n₂ , the we get

1/f =0