We know that for converging lens, $f>0$ , for diverging lens, $f<0$. But for many materials I have read so far, it says that: "the focal length of a concave lens is 8 cm."

I thought that the focal length in this case must be negative. Or when it comes to focal length, we need to find the absolute value of $f$?

  • $\begingroup$ More on sign conventions in optics. $\endgroup$
    – Qmechanic
    Jan 9 '16 at 7:36
  • $\begingroup$ Thank for your reference. I know that for concave lens f<0 . But how about the focal length. Should we take the focal length as the magnitude of f so that the focal length is positive for all lens? $\endgroup$ Jan 9 '16 at 9:17
  • $\begingroup$ I think the magnitude has been quoted. The negative sign is implied. Also, there might be other sign conventions, so leaving off the sign avoids conflicts related to different sign conventions. $\endgroup$
    – garyp
    Jan 9 '16 at 15:07

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