But stalactites point down to the ground!
Oh no, here we go again with another convex/concave kerfuffle.
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When I was little I strictly stuck with c and g indicating ceiling and ground. Simple and easy.
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Mites go up and tights come down!
Helps if you are English to understand that oneā¦
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Old enough to know better. I took that class in my sophomore year at university. 'Nuff said.
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So thatād make you around 420 years old now? (around 20 years old at that time 40 decades ago)
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Thanks! I really wasnāt seeing my typo there. 
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Isnāt this the same man working for the government? Hmmmmmmm 
Contact lenses can be either as far as how they refract light. But generally, for a lens, it is the direction the light is going that determines which surface is convex or concave. As any negative or positive lens can have both convex or concave surfaces. Go 
- Rich
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The lens is neither by itself. Its surfaces are either convex or concave. A plano lens can be convex or concave because thereās only one surface being dimpled (in or out). But contact lenses are a complex shape because the overall lens is by definition concave (to match the curvature of the convex eyeball), except that the optics can require a different shape internal to its thickness so you get something thatās either positive or negative meniscus where the relationship of the convex surface to the concave surface changes the optical property of the lens. The nomenclature is modified in order to get rid of the ambiguity of referential viewing of the lens surfaces due to its thinness.
The only reference needed for a typical convex/concave determination is where the interior of the solid is. Curvature is identified in relation to the objectās center (in thickness). Someone suggested that a cave is concave from outside the cave but to the mountain itās convex which is not correct because the cave is a negative space - the solid to be referenced is the mountain so a cave is always concave and a lava dome is always convex. The mountainās internal perspective is irrelevant.
Otherwise weād always have to provide the reference line when defining a lens or any other concave/convex surface. 
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@jamesdhatch, an even more nerdy (read accurate) response than meā¦cool. Nerds unite! 
Go 
- Rich
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Gotta do something to while away the hours until my GF arrives and I have something to distract me 
Technical accuracy is something that I think is too quickly discarded and leads to lots of misconceptions and innacuracy when itās ignored - the noise around @danās posts are a perfect example. He is extraordinarily clear about something and then people gloss over the detail and create new realities. Gotta drive him nuts.
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Yes! Exactly. Thank you. Go - Rich