ABSTRACT
An optical image is ordinarily taken to be the apparent reproduction of an object formed by an optical system, and is represented by a non-uniformity of light distribution over a receiving surface such as a focusing screen or film. A real image is one that can be projected on a screen, while a virtual image is one formed inside an optical system and is seen by looking into the system. An aerial image is the real image in space. A mirror is made by ‘silvering’ a surface with a suitable coating of a material such as silver or aluminium, and an image is given by reflection. Aspherical mirrors with paraboloidal and ellipsoidal shapes have useful applications, especially in illumination systems, by virtue of their focusing properties for light. Image formation that is limited to small apertures and angles is termed paraxial optics or Gaussian optics.
