Focal length, f = -1/5 = 0.02 m.
Nature of the lens = Concave lens

If object pin is rept exactly at the focus of convex lens rays emerging out of the lens towards the mirror will be parallel. These rays which are perpendicular to surface of plane mirror are reflected along same path, got reflected and form image at F.
 

Lens makers formula: It relates focal length of a lens of the refractive index of material of lens and radii of curvature of two surfaces of lens.  

Let us consider a conves lens of glass having RI n₂ placed in rarer medium of Rln_1. P₁ and P₂ be poles of C₁ and C₂. R₁= (P₁C₁), R₂ = (P₂C₂). τ= P₁P₂(thickness of lens).

Let a point object O be placed on principal axis at a distance is u from P₁. A ray OA travelling in rarer medium is incident on first surface of lens. It is refracted towards normal at A.

If the second surface of lens were absent, the ray AB would have met the principle axis at I’ which is image of V.

Let the distance of I₁ from P₁ be V₁

The roy AB travelling in medium n₂ is actually incident an second surface of lens and refracted into rarer medium n₁ bending away from normal at B. The emergent ray meets principal axis at I which is final real image of O.

For refraction at second surface I acts as vertual image and I is real image, v → distance of I from P₂. 
 

Let the mean refractive index of the material of the lens be n₁ and the mean focal length of the lens be f_y.
From lens makes formule,