A lens is a piece of transparent material that concentrates or disperses light rays when they pass through it by refraction. Lenses are used in various optical instruments such as telescopes, cameras, and microscopes due to their magnifying property. There are different types of lenses available, and they are categorized as simple lenses or compound lenses. The area of the lens suitable for refraction is called the aperture, and the effective diameter of its light-transmitting area is the aperture of the lens. The key features of a lens include:
- Focus: The point onto which collimated light parallel to the axis is focused.
- Focal Length: The distance between the optical center and the focal point or focus of the lens.
- Power: The reciprocal of the focal length of the lens, and the SI unit of power is dioptre.
Lenses can be classified into two types: convex and concave lenses. Convex lenses are also known as converging lenses, and they converge the rays of light that pass through them. Concave lenses, on the other hand, are known as diverging lenses, and they diverge the rays of light that pass through them. When a ray parallel to the principal axis strikes a concave or convex lens, the reflected ray passes through a focus on the principal axis. When a ray passing through the focus strikes a concave or convex lens, the reflected ray will pass parallel to the principal axis. Concave lenses are used in telescopes and spy holes in doors, while convex lenses are used in cameras and magnifying glasses.
