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[45] [47] [48] The English friar Roger Bacon, during the 1260s or 1270s, wrote works on optics, partly based on the works of Arab writers, that described the function of corrective lenses for vision and burning glasses. These volumes were outlines for a larger publication that was never produced, so his ideas never saw mass dissemination.
The prism is also less bulky than the double Porro design. The prism is sometimes simply called a "roof prism", although this is ambiguous, because other roof prisms exist, such as the Amici and Schmidt–Pechan designs. A variant of the Abbe–Koenig prism replaces the "roof" section of the prism with a single mirror-coated reflecting surface.
Bifocals are eyeglasses with two distinct optical powers. Bifocals are commonly prescribed to people with presbyopia who also require a correction for myopia , hyperopia , and/or astigmatism . History
Figure 2. Geometry of a prism compressor Figure 3. Effective pathlength for a prism compressor with A = 100 mm, θ = 55°, and α = 10°. The colors correspond to different values of B, where B = 67.6 mm means that the beam barely hits the tips of both prisms at refractive index 1.6.
If there is refraction at a collective spherical surface, or through a thin positive lens, O'2 will lie in front of O'1 so long as the angle u2 is greater than u1 (under correction); and conversely with a dispersive surface or lenses (over correction). The caustic, in the first case, resembles the sign > (greater than); in the second < (less than).
While in principle aspheric surfaces can take a wide variety of forms, aspheric lenses are often designed with surfaces of the form = (+ (+)) + + +, [3]where the optic axis is presumed to lie in the z direction, and () is the sag—the z-component of the displacement of the surface from the vertex, at distance from the axis.
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