What is the distance between the pole and the principal focus of a spherical mirror called?

In torches,searchlights,and headlights of vehicles,the bulb is placed:

$(a)$ State Snell's law of refraction.
$(b)$ When a ray of light travelling in air enters obliquely into a glass slab, it is observed that the light ray emerges parallel to the incident ray but it is shifted sideways slightly. Draw a ray diagram to illustrate it.

What kind of image is formed when an object is placed between the principal focus $(F)$ and the optical center $(O)$ of a convex lens?

The focal length of a concave mirror is $30 \, cm$. Find the position of the object in front of the mirror so that the image is three times the size of the object.

With the help of a ray diagram,show the formation of an image by a convex lens when the object is placed between $F$ and $2F$. Write two characteristics of the image so formed.