Simple Nature - Light and Matter

j / Spherical mirrors are thecheapest to make, but parabolicmirrors are better for makingimages of objects at infinity.A sphere has equal curvatureeverywhere, but a parabola hastighter curvature at its center andgentler curvature at the sides.through. Either way, we are using a smaller portion of the lens ormirror whose curvature will be more shallow, thereby making theshallow-mirror (or thin-lens) approximation more accurate. Youreye does this by narrowing down the pupil to a smaller hole. Ina camera, there is either an automatic or manual adjustment, andnarrowing the opening is called “stopping down.” The disadvantageof stopping down is that light is wasted, so the image will be dimmeror a longer exposure must be used.k / Even though the spherical mirror(solid line) is not well adaptedfor viewing an object at infinity,we can improve its performancegreatly by stopping it down. Nowthe only part of the mirror beingused is the central portion,where its shape is virtually indistinguishablefrom a parabola(dashed line).What I would suggest you take away from this discussion for thesake of your general scientific education is simply an understandingof what an aberration is, why it occurs, and how it can be reduced,not detailed facts about specific types of aberrations.768 Chapter 12 Optics