What is the $SI$ unit of the power of a lens?

$A$ $10\, mm$ long awl pin is placed vertically in front of a concave mirror. $A$ $5\, mm$ long image of the awl pin is formed at $30\, cm$ in front of the mirror. The focal length of this mirror is ......... $cm$.

Both a spherical mirror and a thin spherical lens have a focal length of $-15 \; cm$. What type of mirror and lens are they?

Is it possible for a lens to act as a convergent lens in one medium and a divergent lens in another?

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

What is the wavelength range of visible light?