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I Is the equivalent lens of two such that f_1+f_2<h divergent?

  1. May 6, 2017 #1
    The focal of the lens equivalent of two thin lens at distance h is
    $$1/f=1/f_1+1/f_2+h/(f_1 f_2)$$

    Therefore, supposing that ##f_1>0## and ##f_2>0## (both lenses are convergent), if ##f_1+f_2 <h## then the equivalent lens should be divergent.

    Nevertheless consider the example in picture

    The two lenses have focals such that ##f_1+f_2 <h## but the image is real, i.e. the equivalent lens cannot be divergent. I understood the ray diagram, but how can this hold true?
  2. jcsd
  3. May 7, 2017 #2
    Can you quote the source for this claim?
  4. May 7, 2017 #3
  5. May 8, 2017 #4
    Maybe the condition for the equivalent lens to be diverging is ##f_1+f_2 >h## . In that case, f1 is to the right of f2 in your diagram. A real image formed by the first lens will be within a focal length of the second lens and the result will be a virtual image.
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