Define power of a lens. Write its S.I. unit

What do you mean by power of a lens ? What is the SI unit of the power of a lens ? Calculate the power of a concave lens of focal length 25 cm.

Define potential gradient. Give its SI unit?

Combination of lenses are used in optical instruments to obtain the required power. What do you mean by power of a lens ? Express its unit.

Define surface energy. What is its SI unit?

Define electric power. What is its Si unit?

What change occurs for power of a lens when its focal length increases ?