Convex (converging. positive) lenses cause parallel rays of light to converge. As you can see, the light rays are bent by refraction, as described, except that the amount of refraction increases from the middle of the lens (where there is no refraction) to a maximum at the perimeter of the lens. The point where the light rays all cross over is known as the principal focus, and the focal length f is the most important feature of the lens. Note that there are two principal foci. (The word ‘foci’ is the plural of ‘focus) The foci are each side of the lens, at the same distance away from it.
The principal axis is the line through the middle of the lens and at right angles to it. The principal foci lie on this line.
Converging lenses are used to form images in magnifying glasses, cameras, telescopes, binoculars, microscopes, film projectors and spectacles for longsighted people.