We define a varied collection of useful terms. Some of these definitions will be revisited when the appropriate topics are taken up.

(i) Ray: The path of light, as determined within the approximations of geometric optics, is a ray. In a homogeneous medium, it is a straight line.

(ii) Beam: A collection of rays, usually referred to as a bundle of rays, forms a beam. One may have a convergent, divergent or, a parallel beam. A convergent beam may converge to a point or, a line, a divergent beam may diverge from a point or, a line. A parallel beam consists of parallel rays.

(iii) Collimation: A process whereby a divergent (or, convergent) beam is rendered parallel usually through the use of lenses and/or mirrors.

(iv) Object & Image: The term object is used to refer to any object (being photographed or observed) that is a source of light, and its likeness (usually two dimensional) formed or observed by an optical system is the image.

(v) Optical System: An optical system consists of elements like lenses, mirrors, prisms, etc.

(vi) Axis: The axis of an optical system is frequently an axis of symmetry such that a ray directed along the axis continues in the same direction or, returns backwards (if reflected within the system)

(vii) Centre of Curvature: Most lenses and curved mirrors being manufactured spherical (i.e., their surfaces are spherical), the centre of curvature of the curved mirror or, the curved surface of a lens is important and is frequently denoted by the letter C. The radius of curvature is also equally important in the analysis.

(viii) Pole: The pole of a spherical surface (refracting or, reflecting) is the central point of the surface involved in the formation of the image. It is denoted by O or, P. The axis, for a single spherical surface, is the join of the pole with the centre of curvature; it is known as the principal axis.

(ix) Optical Centre: The optical centre of a thin lens is a point on the axis of the lens such at a ray directed towards that point emerges parallel to itself after passing through the lens.

(x) Paraxial Rays: It is observed that the formation of clear images by spherical surfaces takes place only with rays which are close to the principal axis and make very small angles with it.

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