write 3 difference between concave and convex lens?
A concave lens is thinnest in its middle (the point where the line between the two focal points intersects the lens (at right angles to the plane of the lens).
A convex lens is thickest in its middle.
A convexo-concave lens is concave on one side and convex on the other. Virtually all spectacle (eye glasses) lenses are like this. Whether the particular lens is convex or concave is answered by the thickness at it middle compared with elsewhere on the lens.
You can also consider all the combos: single (the other side is flat), double convex and concave.