Lenses are used to make light converge or diverge, that is to concentrate it to a point or spread it out. In science they can be used to make telescopes and magnifying glasses, used with rayboxes to calculate focal lengths and help to understand how the eye works.
Most lenses are made from shaped glass and are easily chipped if (when) dropped so take care to keep them in some kind of padded box separated by cloth or foam. Some companies sell plastic lenses which when new are pretty identical to the glass varieties however over time they can get scratched which will shorten their useful lifespan.
The main types of lenses used in school science are pictured in fig 1 below.
Fig 1: Lens types
Plano-convex and biconvex lenses enable light beams shining parallel through the lens to converge or focus at a point behind the lens. The distance between this point and the centre of the lens is called the FOCAL LENGTH (see fig 2 below).
Fig 2: Converging lens
Convex-concave lenses work in the opposite way. The parallel light beams diverge and therefore the focal point lies at a point on the other side of the lens (see fig 3 below).
If a meniscus lens is used (both curvatures are equal) then the beam is not converged or diverged.
Fig 3: Diverging lens
Fresnel lenses consist of a stepped glass surface texture. They have been used in lighthouses and theatre lanterns for many years and feature because of one important property. They are stepped so large lenses can be constructed which have the same focal lengths and apertures of traditional plano-convex lenses but use less material and are therefore cheaper and lighter. Fig 4 shows the difference between a fresnel and a standard plano-convex lens. The stepped textured glass pieces are known as fresnel zones and work like many prisms in unison to focus the light.
Fig 4: Fresnel and plano convex lenses
The lenses we have in our eyes are biconvex structures which help focus light onto the retina. The lens is soft and can change shape with help from tiny muscles which hold it in place. By changing the curvature of the lens, we can focus on objects at varying distances. This process is called accommodation.
Because the lens continues to grow and toughen throughout our lives, when we get into old age the focusing abilities deteriorate.