What is Lens ? What are types of lenses ?

What are the types of lens?

What is a lens ? Name the two types of lenses. Name any five things which use lenses.

When we see the eyes of a person through the lenses of spectacles, work by him, they looked small in size ? What types of lens are they ?

Doctor has presecribed a lens having power + 1.5D. What will be the focal length of the lens ? What is the type of the lens and what must be the defect of vision ?

Doctor has prescribed a lens having power +2 D . What will be the focal length of the lens ? What is the type of the lens and what must be the defect of vision ?

Doctor has prescribed a lens having power +1.5 D . What will be the focal length of the lens? What is the type of the lens and what must be the defect of vision?