Multiple images are formed by a thick plane mirror. Which of the following is the brightest of all the images?

$(a)$ Define principal focus of a spherical mirror.
$(b)$ For what position of the object does a concave mirror form a real,inverted and diminished image of the object? Draw the ray diagram.
$(c)$ An object $4 \,cm$ high is placed at a distance of $6 \,cm$ in front of a concave mirror of focal length $12 \,cm$. Find the position of the image formed.

If the radius of curvature of a concave mirror is $36\, cm$,what is its focal length (in $, cm$)?

The magnification equation is a relation between image distance and object distance.

What is the power of a concave lens of focal length $25\, cm$?

Through which point of a convex lens does a ray of light pass without undergoing refraction?