![]() The diffraction integrals can be computed for the optical system that includes the focusing mirror. The question arises, what is the intensity of the light at the focal point for a perfectly focused beam? Does the intensity approach infinity for a perfect parabola? The answer is supplied by diffraction theory. This means that constructive interference will occur at the focal point from a diffraction theory standpoint.) What are the focused spot size and focused beam intensity in the case of perfect focusing? Using this, you can readily show that the path lengths to the focal point are identical for each of the rays at the focal point. ![]() (You may recall from algebra classes that one definition of a parabola is the locus of points that is equidistant from a focus and a line called the directrix. Even a simple diffraction theory-based calculation shows that the path distance to the focal point is identical for each of the rays. When a collimated beam is brought to a perfect focus by a paraboloidal mirror, the focusing is essentially perfect in the sense that when ray-tracing is used with the angle of incidence equal to the angle of reflection, all of the parallel on-axis rays converge precisely to the focal point. The reader is assumed to be somewhat familiar with the concepts of diffraction theory and the Huygens sources that are used to compute a diffraction pattern. ![]() The purpose of this Insights article is to give the reader a brief introduction to the principles behind diffraction-limited focusing. Comparison with spherical lenses and mirrors=the case of imperfect focusing:.What are the focused spot size and focused beam intensity in the case of perfect focusing?.Perfect Focusing by paraboloidal mirror:.
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