# Polishing Spherical Refracting surface vs Lens

My text book states that

When we polish a spherical refracting surface (sphere) it doesn't behave as spherical surface and mirror .It still behaves as spherical surface. The polished side is just use to change the direction of light.

My question is Why then when we polish a lens it behaves as lens and mirror and not only as lens.Ex - Concavo Convex