Refraction by Spherical Lenses

"A lens is a transparent material bounded by two surfaces, at least one of which is spherical, that refracts light to form an image."

"A convex lens is a lens that has two spherical surfaces bulging outward (double convex), and it converges light rays passing through it."

"A concave lens is a lens bounded by two spherical surfaces curved inward (double concave), which is thicker at the edges than at the middle, and diverges parallel light rays passing through it."

An imaginary straight line passing through the two centres of curvature of a lens is called its principal axis.

The central point of a lens is its optical centre. It is usually represented by the letter O.