light ray

Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by aberrations of the imaging optics.

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In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge.

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A real image occurs where rays converge, whereas a virtual image occurs where rays only appear to diverge.

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In ray diagrams ... real rays of light are always represented by full, solid lines; perceived or extrapolated rays of light are represented by dashed lines.

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A real image ... is an image which is located in the plane of convergence for the light rays that originate from a given object.

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A converging lens (one that is thicker in the middle than at the edges) or a concave mirror is also capable of producing a virtual image if the object is within the focal length. Such an image will be magnified.

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A diverging lens (one that is thicker at the edges than the middle) or a convex mirror forms a virtual image. Such an image is reduced in size when compared to the original object.

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Because the rays never really converge, a virtual image cannot be projected onto a screen. In contrast, a real image can be projected on the screen as it is formed by rays that converge on a real location.

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In diagrams of optical systems, virtual rays are conventionally represented by dotted lines.

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a virtual image is found by tracing real rays that emerge from an optical device (lens, mirror, or some combination) backward to perceived or apparent origins of ray divergences.

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