A spherical lens is a transparent medium bounded by two spherical caps or by a spherical cap and a plane. There are two types of lenses: thin-edged (converging) lenses and thick-edged lenses.

pH (potential of hydrogen) is a physical quantity which allows us to measure the acidity or basicity of a solution. --if the pH<7 the solution is acidic (more H+ ions) --if the pH=7 the solution is neutral (H+=HO-) --if pH>7 basic solution (more HO- ion)