Why mirror do has one principal focus and lens has two principal foci?
Answers
This is because mirror performs reflection and lens refraction. Mirror sends the light ray back, so it has one principal focus while lens, having to bend the light ray lets the light ray pass through the second principal focus to form an image.
mirror has only 1 focus as the incident ray intersects only 1 side but in case of a mirror and that side only acts as the reflecting surface.
when we consider the case of a lens , a lens is made up of two such transparent mirrors , hence when we consider its principle focus both of them are considered. also if the lens has only one focus then a ray diagram is not possible as when the incident ray is at infinity then the light rays cannot converge at the focus of the lens( which is before the lens or on its left side) as the lens undergoes refraction and not reflection.