Why electromagnetic waves reflect at junction points of cables

         Z1                 Z2
  ------------------o-------------------
      (Vi, Ii) ->       -> (Vt, It)
      (Vr, Ir) <-

Fig. 1. Reflection at junction point of wires with different characteristic impedance

When electromagnetic waves propagating toward a junction point of wires with different characteristic impedances reach the junction point, it is a well known fact that some or all of the waves reflect and the rest is transmitted.

For example, when the following variables are defined in Fig. 1,

  Vi = Voltage of electromagnetic wave toward junction point (V)
  Ii = Current of electromagnetic wave toward junction point (A)
  Vr = Voltage of electromagnetic wave reflected at junction point (V)
  Ir = Current of electromagnetic wave reflected at junction point (A)
  Vt = Voltage of electromagnetic wave transmitted through junction point (V)
  It = Current of electromagnetic wave transmitted through junction point (A),

many literature show the following result.

  Vr/Vi = (Z2 - Z1)/(Z2 + Z1)
  Vt/Vi = 2*Z2/(Z2 + Z1)

Vr/Vi is called the voltage reflection coefficient, and Vt/Vi is the voltage transmission coefficient.

Many people "know" these relations, but when I asked the question, "why do reflections happen?" to one of such people, this person started to brood over the question.

This experience led me to think that this type of question might be necessary.

The question is why electromagnetic waves reflect at the junction point of wires with different characteristic impedance.

This should be too trivial a question to most people, but...

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