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Stargazing Why isn't the magnification of a telescope (-)

  1. Dec 7, 2007 #1
    I was wondering why the magnification of a telescope is not considered negative since it is inverted. I know it doesn't really matter much since all you are trying to do is get a larger image but the sign convention should still be followed I think. The book I am using is Giancoli Physics for Scientists.
  2. jcsd
  3. Dec 8, 2007 #2
    A telescope is working with two sections, the objective and the eyepiece. The objective provides an inverted real image of angular magnification(-M). The eyepiece is a magnifier creating a magnified virtual image from the object which is the image of the objective which is upright or +m as it's also an angular magnification. The result is mM = -fobj/fe which is inverted and hence negative. When dealing with a 'grocery store' gift telescope (the optical equivalent of a fruitcake) magnification is the most important sales gimmick and having a - sign in front of the magnification would terribly confuse all customers who might consider purchasing one of those toys. Besides, if you insert a right angle mirror or prism star diagnonal, the image becomes upright without affecting the magnification.

    I leave it to your choice concerning why the sign convention in the Giancoli book was not followed as there are possibilities as to why normal sign convention was not followed in that book. Other books such as University Physics texts that introduce the basics of optics - such as Sears & Zemansky (now known as Young & Friedman) do follow the proper convention. Other less important conventions - such as s and s' versus p and q for object and image distances or M and m for angular versus lateral magnification (or lateral versus angular magnification) do tend to vary from textbook to textbook, perhaps to avoid potential copyright infringement. Note here I'm using both m and M for angular magnification - just for the eyepiece and objective rather than trying to figure out how to insert subscripts to differentiate them (at 4am local time).
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