A "bel",  [1]https://en.wikipedia.org/wiki/Decibel    is a logarithmic
   measure of relative amplitude, most commonly used when values vary by
   many factors of 10.  While most people think of it in terms of sound
   intensity, it is also a description of radio signal strength, antenna
   gain, and other values.
   a "3 decibel" difference is approximately a factor of 2 in power.   So,
   if an optical fiber has an optical loss of "0.15 dB/kilometer", that
   means that 20 kilometers of fiber has about 20 kilometers x 0.15
   dB/kilometer = 3 dB loss, or a factor of 2 in power.
   --
   Despite the fact that I had absolutely nothing to do with selecting my
   name, I can manage a minor gloat by saying that I am one of  few people
   whose name is also the name of an ASCII character:
   [2]https://en.wikipedia.org/wiki/ASCII
   (Jim) Bel(l)
   Tab (Hunter)
   Del  (Webb, or Shannon)

   On Sunday, January 5, 2020, 02:06:11 PM PST, \0xDynamite
   <dreamingforward@gmail.com> wrote:
   > But even at about 1550 nanometers, 50 kilometers of fiber has:  50 km
   x 0.15 dB/km  = 7.5 dB of optical loss
   Isn't there a better unit than Bells (dBs) for optics, like
   transparency or opacity?  It seems Bells are directly related to air
   pressure....
   Mark

References

   1. https://en.wikipedia.org/wiki/Decibel
   2. https://en.wikipedia.org/wiki/ASCII