The radius of the sphere whose part is the lens surface is called the …………… of that surface.

1. centre of curvature
2. focal length
3. pole
4. radius of curvature

Identify the incorrect statement :

1. if the medium on both sides of a lens is same, its first and second focal lengths are equal, i.e. f₁ = f₂ (numerically).
2. when we say focal length of a lens, we mean the first focal length of the lens.
3. a convex lens has a real focus.
4. a concave lens has a virtual focus.

Which lens is converging —

(i) an equiconcave lens or an equiconvex lens?

(ii) a concavo-convex lens or a convexo-concave lens?

State the condition when a lens is called equiconvex or equi-concave.