Calculate the radius of curvatuire of the concave lens based on the measured focal length.

 R₁ = (n -1) f

Explanation:

In geometric optics the focal length and the radius of curvature are related, for the case of a lens

          1 / f = (n₂-n₀) (1 / R₁ - 1 / R₂)

where f is the focal length, n₂ is the refractive index of the material, n₀ is the refractive index of the medium surrounding the material, R₁ and R₂ are the radius of curvature of each of the material's

In our case, the most common is that the lens is in the air, so n1 = 1, in many cases one of the surfaces is flat, so its radius of curvature R₂ = ∞.

     1 / f = (n-1) (1 / R₁)

we look for the radius of curvature R₁

    1 / R₁ = 1 / f (n-1)

     R₁ = (n -1) f

With this expression we can find the radius of curvature of a concave-plane lens

d is the correct answer