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.

A convex lens of focal length f is divided into two equal parts by a plane parallel to the principal axis. The focal length of each part will be :

1. 2f
2. f/2
3. f
4. infinite

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.