Here is more on the type of lens he made called a fresnel lens from wikipedia:

A Fresnel lens (pronounced /freɪˈnɛl/ fray-nel or /ˈfrɛznəl/ frez-nel) is a type of compact lens originally developed by FrenchphysicistAugustin-Jean Fresnel forlighthouses.^[1]

The design allows the construction of lenses of large aperture and short focal length without the mass and volume of material that would be required by a lens of conventional design. A Fresnel lens can be made much thinner than a comparable conventional lens, in some cases taking the form of a flat sheet. A Fresnel lens can capture more oblique light from a light source, thus allowing the light from a lighthouse equipped with one to be visible over greater distances.

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