A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. Lenses have curved surfaces, or a very large number of flat surfaces located at slightly different angles.
There are many types of lenses. They differ from one another in terms of their shape and the materials from which they are made. We focus on lenses that are symmetrical across their horizontal axis – known as the principal axis.
- A Converging lens is a lens that converges rays of light that are traveling parallel to its principal axis. Converging lenses can be identified by their shape; they are relatively thick across their middle and thin at their upper and lower edges.
Converging lenses (positive lenses) are thicker at the centre than at the edges.
- A Diverging lens is a lens that diverges rays of light that are traveling parallel to its principal axis. Diverging lenses can also be identified by their shape; they are relatively thin across their middle and thick at their upper and lower edges.
Diverging lenses (negative lenses) are thicker at the edges than at the centre.
- A convex lens can forms real and inverted image. When the object is placed very close to the lens, the image formed is virtual, erect and magnified. When used to see objects magnified, the convex lens is called a magnifying glass.
- The principal focus (F) is a point on the principal axis where light comes to a focus (for a converging lens) or appears to be diverging from (for a diverging lens). Two foci exist, equidistant on either side of the lens, since light behaves the same way when travelling in either direction (Principle of Reversibility). The two foci, F and F’ are called the primary principal focus and the secondary principal focus, respectively. F, sometimes also referred to as the primary focal point, is shown on the right side of a converging lens, and on the left side of a diverging lens, while F’, the secondary focal point is shown on the opposite side of each respective lens.)