Why a concave lens has a real principal focus .while convex mirrors has a virtual principal focus
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THE PRINCIPAL FOCUS
The principal focus is defined as a “point on the principal axis”through which “paraxial rays will pass”. The paraxial rays are nothing but the rays which are parallel to the principal axis.When considering the case of only mirrors, then very approximately, the radius of curvature equals the focal length.
INTUTION TO CONVEX MIRRORS
The convex mirrors are basically diverging(rays) mirrors. We also know that, to see an image, we require at least 2 light rays (transmitted or reflected).Therefore, when the paraxial rays are made to travel towards this convex mirror, all the light rays after reflecting ‘appear’ to come from the principal focus.
VIRTUAL PRINCIPAL FOCUS
Since the principal focus is behind the mirror, that focus is called as virtual principal focus, for convex mirror.
The principal focus is defined as a “point on the principal axis”through which “paraxial rays will pass”. The paraxial rays are nothing but the rays which are parallel to the principal axis.When considering the case of only mirrors, then very approximately, the radius of curvature equals the focal length.
INTUTION TO CONVEX MIRRORS
The convex mirrors are basically diverging(rays) mirrors. We also know that, to see an image, we require at least 2 light rays (transmitted or reflected).Therefore, when the paraxial rays are made to travel towards this convex mirror, all the light rays after reflecting ‘appear’ to come from the principal focus.
VIRTUAL PRINCIPAL FOCUS
Since the principal focus is behind the mirror, that focus is called as virtual principal focus, for convex mirror.
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