Does a reverse telescope magnify or minify objects?

• A. Magnify
• B. Minify
• C. Neither
• D. Both

Answer: (B)

The key idea is how angular magnification depends on the two lenses’ focal lengths and what happens when you swap their positions. In a normal telescope, the objective forms a real image at its focal plane, and the eyepiece magnifies that image, giving a large apparent angular size. The magnification is essentially the ratio of the objective’s focal length to the eyepiece’s focal length.

If you reverse the telescope, the roles swap. The lens that used to be the eyepiece now acts as the objective, and the former objective becomes the eyepiece. The overall angular magnification becomes the reciprocal of the original: fe/fo. Since the objective’s focal length is typically longer than the eyepiece’s, this ratio is less than one. That means the image you’d view through the reversed telescope subtends a much smaller angle than the object does, i.e., it minifies.

So, reversing a telescope yields minification, not magnification (and certainly not both or neither).