That is, you can have a sphere and a parabola that have the same first and second derivative, and they will differ only in third order (in fact, since they're both even function, they will differ in the fourth order terms). I don't know whether you've taken Calculus yet, but in Calculus terms, spheres and parabolas are second order approximations to each other. Other kinds of telescopes use spherical mirrors, but correct the spherical aberration with lenses or other optical elements.Įither there was some qualification that you missed, or your physics class is being overly simplistic. Small Newtonian telescopes, commonly around 114 mm diameter and 900 mm focal length, usually have spherical mirrors and are diffraction limited or nearly so. If a spherical mirror is a small enough section of a sphere of large enough radius, then it can still be diffraction limited. If the spherical aberration causes less image degradation than diffraction, then little or nothing is gained by using a parabola, which is harder to make. In reality, all optics suffer from diffraction. ![]() And parabolic telescope mirrors look spherical and very nearly are spherical, deviating from the sphere by perhaps only millionths of an inch. Telescope mirrors are much less curved, almost flat. Now, the question arises: if parabolic mirrors are more efficient than spherical mirrors, why even make spherical ones?įor optical applications, like Newtonian telescopes, the illustrations here are greatly exaggerated. You can see multiple focal points in concave one, whereas a single focal point in the parabolic one. When rays hit the mirror far from principal axis they create different focal point creating multiple focal points, collectively known as focal volume. Spherical mirrors also have one focal point only when the rays coming are paraxial (rays very close to principal axis). The only difference between them is that parabolic mirrors are more precise they have only one focal point. ![]() There are both spherical and parabolic mirrors. Well, the mirrors you are learning in physics are spherical.
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