What is the point,where the rays parallel to the principal axis meet after reflection,called?

Which of the following statements is true about the image formed by a plane mirror?

How many centres of curvature does a lens have?

Identify the device used as a spherical mirror or lens in the following cases,where the image formed is virtual and erect in each case.
$(a)$ Object is placed between the device and its focus,image formed is enlarged and behind it.
$(b)$ Object is placed between the focus and the device,image formed is enlarged and on the same side as that of the object.
$(c)$ Object is placed between infinity and the device,image formed is diminished and between the focus and optical centre on the same side as that of the object.
$(d)$ Object is placed between infinity and the device,image formed is diminished and between the pole and focus,behind it.

In which medium is the speed of light the highest?

Beams of light are incident through the holes $A$ and $B$ and emerge out of the box through the holes $C$ and $D$ respectively,as shown in the figure. Which of the following could be inside the box?