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

An object of height $5 \, cm$ is placed perpendicular to the principal axis of a concave lens of focal length $10 \, cm$. If the distance of the object from the optical centre of the lens is $20 \, cm$,determine the position,nature,and size of the image formed using the lens formula.

Can a plane mirror be called a spherical mirror?

Give the mathematical expression for the power of a lens.

What is meant by the power of a lens? Give its $SI$ unit. When two or more lenses are placed in contact,what will be their combined power?

What is the mirror formula for a spherical mirror?