"Joe" wrote in message

The plano lens is either convex-concave on one side and Plano Flat on
the other.

Not quite. IF concave on one side and flat on the other, it's a minus power
lens. IF convex on one side and flat on the other, it's a plus power lens.
The only way a plano lens can be curved is for it to be convex on one side
and concave on the other. The curvature of each respective side will depend
upon the index of refraction of the lens material in order to remain plano
in power.

If just flat then it is not a lens. . That simple Max.

Hardly. Any plano-plano transparent media can become a lens, dependent upon
its usage. For example, a neutral density lens for a camera is almost
always plano-plano, as are colored filters for the same purpose. All are
correctly termed lenses.

Do you really want to continue this debate? You're out of your league here.

Max