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A lens is conventionally defined as a piece of glass which is used to focus or change the direction of a beam of light passing through it. They are mainly made of glass or plastic. Lens are used in making spectacles, cameras, cinema projectors, microscopes and telescopes.
Types of thin lenses.
A lens which is thicker at its centre than at its edges converges light and is called convex or converging lens. A lens which is thicker at its edges than at its centre diverges light and is known as concave or diverging lens.
Properties of lenses.
- Optical centre – this is the geometric centre of a lens which is usually shown using a black dot in ray diagrams. A ray travelling through the optical centre passes through in a straight line.
- Centre of curvature – this is the geometric centre of the circle of which the lens surface is part of. Since lenses have two surfaces there are two centres of curvature. C is used to denote one centre while the other is denoted by C1.
- Principal axis – this is an imaginary line which passes through the optical centre at right angle to the lens.
- Principal focus – this is a point through which all rays travelling parallel to the principal axis pass after refraction through the lens. A lens has a principal focus on both its sides. F is used to denote the principal focus
- Focal length – this is the distance between the optical centre and the principal focus. It is denoted by ‘f’.
The principal focus for a converging lens is real and virtual for a diverging lens. It is important to note that the principal focus is not always halfway between the optical centre and the centre of curvature as it is in mirrors.