Which mirror is used by dentists?

$A$ convex mirror used on an automobile has $3\, m$ radius of curvature. If a bus is located at $5\, m$ from this mirror,find the position,nature,and size of the image.

Where should an object be placed to use a convex lens as a simple microscope?

When is the magnitude of the power of a lens equal to its focal length?

What is the power of a convex lens with a focal length of $20 \text{ cm}$?

An object is placed at a distance of $25 \, cm$ away from a converging mirror of focal length $20 \, cm$. Discuss the effect on the nature and position of the image if the position of the object changes from $25 \, cm$ to $15 \, cm$. Justify your answer without using the mirror formula.