Lens :A lens is a transparent refracting medium bounded by two curved surfaces which are generally spherical.
Uses of a convex lens
- It is used in a number of optical instruments such as camera, telescope, microscope and so on.
·It is used as a reading lens and as a magnifying glass.
· It is used in spectaclesfor the correction of long-sightedness of the eyes.
Uses of a concave lens
- It is used in telescopesto see far off places.
· It is used in spectacles for the correction of the short-sightedness of the eyes.
The centre of curvature of a lens is usually represented by the letter C. Since there are two centres of curvature, we may represent them as C1 and C2..The radius of the sphere, of which the part is the lens surface is called the radius of curvature of that surface of the lens.An imaginary straight line passing through the two centres of curvature of the lens is called itsprincipal axis.
The central point of a lens is its optical centre.It is usually represented by the letter O.A ray of light through the optical centre of a lens passes without suffering any deviation.The effective diameter of the circular outline of a spherical lens is called its aperture.A lens has two principal foci F1 and F2. F1 is the first focal point and F2 is the second focal point.The distance of the principal focus from the optical centre of a lens is called its focal length.