A doctor has prescribed a corrective lens of power, $-1.5 \mathrm{D}$. Find the focal length of the lens. Is the prescribed lens diverging or converging?

A convex lens of power $5 \mathrm{D}$ and a concave lens of power $7.5 \mathrm{D}$ are placed in contact with each other. What is the:

Power of this combination of lenses? 

A convex lens of power $5 \mathrm{D}$ and a concave lens of power $7.5 \mathrm{D}$ are placed in contact with each other. What is the:

Focal length of this combination of lenses?

A convex lens of focal length $25 \mathrm{cm}$ and a concave lens of focal length $10 \mathrm{cm}$ are placed in close contact with one another.

What is the power of this combination?

A convex lens of focal length $25 \mathrm{cm}$ and a concave lens of focal length $10 \mathrm{cm}$ are placed in close contact with one another.

What is the focal length of this combination?