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

What is the focal length of a convex lens having power $+5.0 \,D$?

$A$ $4.5 \, cm$ needle is placed $12 \, cm$ away from a convex mirror of focal length $15 \, cm$. Give the location of the image and the magnification. Describe what happens to the image as the needle is moved farther from the mirror.

If the image formed by a mirror for all positions of the object placed in front of it is always erect and diminished,what type of mirror is it? Draw a ray diagram to justify your answer. Where and why do we generally use this type of mirror?

What type of lens behaviour will an air bubble inside water show?

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?