State the position and nature of the image formed by a convex lens for the following position of the object:

Between $\mathrm{F}$ and $2\mathrm{F}$

State the position and nature of the image formed by a convex lens for the following position of the object:

At $2\mathrm{F}$

State the position and nature of the image formed by a convex lens for the following position of the object:

Beyond $2\mathrm{F}$

Explain the following with the help of a ray diagram, in relation to the two main types of spherical lenses:

(a) Principal focus
(b) Focal length