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geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge.[1] Although the focus is conceptually a point, physically the focus has a spatial extentFor a lens, or a spherical or parabolic mirror, it is a point onto which collimated light parallel to the axis is focused. Since light can pass through a lens in either direction, a lens has two focal points—one on each sideElliptical mirrors have two focal points: light that passes through one of these before striking the mirror is reflected such that it passes through the other.The focus of a hyperbolic mirror is either of two points which have the property that light from one is reflected as if it came from the other.A diverging (negative) lens, or a convex mirror, does not focus a collimated beam to a point. Instead, the focus is the point from which the light appears to be emanating, after it travels through the lens or reflects from the mirror.
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what is meant by first principle focus
