Ask A Scientist© Physics Archive

name         Kevin D.
status       student
age          20s

Question -   How exactly is the speed of light slowed down as
it travels through a given medium?
Does the speed of light actually remain constant and the medium
forces the light to travel in a sort of zig-zagged path whereby it simply
travels a further distance and thus it "seems" to be slowed down?
Or do the photons interact in some way with the surrounding particles
such that the actual speed of light is slowed down? What
characterizes this interaction? Friction? Refraction?
------------------------------------------------
Hi, Kevin !!

Considering that the light consists of electromagnetic waves,
there will be an interaction with the electromagnetic nature
of the atoms through them the light travels. This phenomenon
is called refraction, and slows down the speed of light. Depending
on the wave length, this interaction will be different. For small
wave lengths the effect is greater than by that of greater wave
lengths. Atoms consist of electrons that are in movement and
thus generate magnetic fields. Depending on the number of
atoms to the crossed, this effect can be greater, for instance,
in solid matter than in liquid or gases. The light doen´t travel
in a criss-cross fashion inside a body but interacts uniformly
with the electromagnetic field of it.

Alcir Grohmann
Beschaffung SAM
=========================================================
JC Maxwell (the father of electromagnetics, in my opinion) will probably
rollover in his grave after this explanation...but here goes.
Light will actually travel much closer to 'c' (speed of light in free space)
than electrons will on most dielectric media.

With electrical circuits, the speed of the travelling electrons is governed,
primarily, by the insulating material surrounding your conductor.  This
speed, also called velocity factor have the following common values of
propagation velocity (Vp) in coaxial cable ranging from 0.66c to 0.84c.  The
reason for this reduction in Vp is due to the insulating materials per unit
length capacitance and inductance of the line.  Remember that the electrical
signal travelling in the conductor is time varying and its conduction
throughout the conductor will be impeded.  Does it follow a sort of zig
zagged path?  To ask if the electrons will actually move slightly from the
left to slightly to the right and so on to reduce its propagation time...the
answer would have to be NO.  The electrons experience this sluggishness due
to the reactance in the coaxial line (capacitance and inductance).

In the same manner, photons will also experience this same sluggishness in
air.

Vp = 1 / (square root of (LC))
L = inductance / unit length of air
C = capacitance / unit length of air

Overall, I do believe you are on to something when you propose that the EM
wave's natural speed is the "speed of light", "c".....because why else would
a radio signal leave the Earth and travel at slightly less than the speed of
light, in air, and then "accelerate" to the free space value for the speed
of light while outside the atmosphere?.

If you care to look over some of the fruits of Maxwell's work (e-mag
equations) follow this link:

http://www.mmwt.com/fiber_optics_microwave_wireless_rf_FRMSet.shtml?fiber_op
tics_wireless_microwave_rf_RC3formulas.shtml&2

If you wish to learn more.  I recommend buying the following book;

Engineering Electromagnetics
by:  Umran S. Inan and Aziz S. Inan

-Darin Wagner
=========================================================