Attenuation is the (usually) unwanted loss of signal due to
    resistive heating or
    radiation of the signal from its medium
    magnetic energy losses in the iron or ferrite cores of inductors
    electrostatic energy losses in the dielectric medium in capacitors

    In fibre optic systems, light can be scattered by imperfections in the glass
    The glass will not be perfectly transparent.

Attenuation makes a strong signal weaker.

Attenuation can be useful if you need to process a strong signal that would otherwise overload your circuit.

A potential divider can be used as an attenuator.

Attenuation is usually a problem because it prevents weaker signals arriving at their destination.

Attenuation in cables is caused by energy loss due to

  • the resistance of the wire (this causes energy loss as heat P = I2R)
  • radiation from the cable (this is a problem with higher frequency signals because the cable starts to act like an antenna)
  • cross talk (the signal leaks from one cable to another due to the capacitance between the wires and due to magnetic coupling)
  • skin effect on Wikipedia. At higher frequencies, the current in a conductor prefers to flow in the surface or skin of the conductor. Since the inner core is not carrying the current, the effective resistance is higher. This causes greater energy loss and attenuation. Litz wire has many thin strands. This gives a higher surface area and the skin effect is reduced. RF burns are caused by high frequency radio energy heating the victim's skin. This is an unpleasant example of the skin effect.

 

Attenuation in optical fibres is caused by energy loss due to

  • imperfections in the glass reflecting light in the wrong direction. This is called scattering.
  • less than perfectly transparent glass.
  • dirt or poor polishing at the fibre connectors.

 

Attenuation in free space is caused by

  • the inverse square law (if you are transmitting in all directions equally and you double your distance from the transmitter, the energy is spread out over the surface of a sphere with four times the surface area so you only receive a quarter of the energy)
  • obstructions between the transmitter and receiver
  • the signal can no longer be received when it becomes significantly weaker than the background noise