Question

define : principal focus ( very shortly)​

Answer 1

Answer:

In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle.

Explanation:

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Answer 2

Answer:

All the incident rays that are parallel to principal axis which converges at a single piont .These point is called as Principal Focus