Question
Fri April 27, 2012 By: Tabish

focus and principal focus

Expert Reply
Mon April 30, 2012
Consider an object and its image. Now a point on the object has an image point on the image. This point image of a point of the object is called focus. The rays of light from the point of the object converge to or appear to diverge from the focus point.

Principal focus is a special case of focus. It is defined by objects at infinite distance. When an object is at infinite distance, rays from a point of the object is parallel and a parallel beam of light which is parallel to the principal axis and close to the principal axis converge to or appear to diverge from a fixed point on the principal axis. This point is called principal focus.
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