# Reflection and Transmission Speed of Electromagnetic Waves

```            |..........
I     |...T......
--------->|------>...
<--|..........
n1   R |....n2....
|..........
```
A ray of light (electromagnetic waves) (I) that vertically comes to a medium with refractive index, n2, from a medium with refractive index, n1, partly makes reflection and the rest passes through. It is well known that the reflection constant, namely the ratio of the reflected waves to the incident waves, can be expressed as follows, when compared in terms of volume of the electric field (note 1):

```  Er/Ei = (n1 - n2)/(n1 + n1)                                     (1)

where,
Ei = Strength of the electric field of incident light (V/m)
Er = Strength of the electric field of reflected light(V/m)
n1 = Refractive index of medium１
n2 = Refractive index of medium２
```
The value has to be squared when compared in terms of the strength (energy) of light. In case of incidence from air to water, for example, the reflexibility is approximately (1/7)^2 = 2 %, where n1 = 1 and n2 = 1/7, and most of the light passes through. This is the reason why water looks transparent.

Now, the (absolute) refractive index is defined as follows:

```  n = v /c

where,
n = Refractive index
v = Propagation speed of electromagnetic waves in the medium(m/s)
c = Light velocity in a vacuum (3e8 m/s)
```
So, the expression (1) can be rewritten as follows:

```  Er/Ei = (v1 - v2)/(v1 + v2)                                      (2)

where,
v1 = Propagation speed of medium１(m/s)
v2 = Propagation speed of medium２(m/s)
```
In others words, there is no reflection if propagation speeds of two mediums are equal. If currents on both sides of the boundary have the same speed, everything that has reached the boundary flows away through the boundary: that is a conclusion easy to understand intuitively.

Now, here is the question. The relation between reflection and permeation of electromagnetic waves at a connecting point of cables looks quite similar to the above. Is it right to think in this case too that, if propagation speeds on both sides of the connecting point are equal, there is no reflection?

```  -------------o--------------
I      |   T
--------->|------>
<--|
v1       R |     v2
-------------o--------------
cable1        cable2
```
If not, how is it different from the aforementioned case of light?

## Note 1 - Reflection of Light

This matter is mentioned in many books. Read the following books, for example. They are masterpieces with full of excitement.

```Richard P. Feynman,- Robert B. Leighton and Matthew Sands - The Feynman
Lectures on Physics, Vol.1, 3306

Richard P. Feynman,- Robert B. Leighton and Matthew Sands - The Feynman
Lectures on Physics, Vol.2, 33-5

Vernon.D.Barger and Mrtin.G.Olsson,- Classical Electricity and Magnetism
A contemporary Perspective, 9-54
(Allyn and Bacon) ISBN 0-205-08758-2
```