What is meant by power of a lens ? What is one dioptre ?

What is meant by power of a lens? What is its unit?

What is meant by number of lens?

What is meant by power of a lens ? When is it positive or negative? Define one dioptre.

What is meant by power of a lens ? Write its Sl unit. A student uses a lens of focal length 40 cm and another of - 20 cm. Write the nature and power of each lens.

What is meant by power of lens? What is SI units? Define one dioptre.