From: Ronald Wong (ronwong@inreach.com)
Date: Mon Dec 13 2004 - 10:18:56 PST
Message-Id: <l03102800bdddc97c0790@[209.209.19.138]> Date: Mon, 13 Dec 2004 10:18:56 -0800 From: Ronald Wong <ronwong@inreach.com> Subject: Re: light antenna
Eric Friedman (mcelover@yahoo.com) asked:
A.
>what exactly is a light antenna?
B.
>...
>didn't edison invent a visible light transmitting antenna when he finally
>made a fillament?
C.
>or are we talking about some kind of coherent, modulated visible light?
The best way to answer A is to answer B.
Question B: Is the filament invented by Edison (but probably discovered by
one of the many minions who worked under him) a "light transmitting
antenna"?
To answer Question B, we have to address two more questions:
1. What is a transmitting/receiving antenna?
Traditionally, the terms "transmitting and receiving antennas"
have been associated with the sending and receiving of radio
waves, tv signals, and data (satellites, microwave, wi-fi, etc.).
The antenna in all these cases was nothing more than a length
of conductor in the form of a wire or rod.
Conductors have free electrons.
First a little review: Air molecules moving freely about in a
tube will suddenly move in an organized fashion when they
are disturbed in a periodic manner by something with the
right frequency. The frequency is based on the tube's
dimensions and the phenomena is called resonance (think:
organ pipes, trumpets, flutes, the sounds you hear when
you put your ear to a conch shell, etc.).
The elections in an antenna will do the same thing if an
electromagnetic wave of just the right frequency is applied to
it. As with the air molecules in a tube, the frequency is based
on the dimensions of the antenna.
There are two ways an electromagnetic wave can be applied to the
antenna.
One way is to apply an alternating voltage to the antenna by
hooking it up to an electronic device (commonly known as a
transmitter) set to this frequency. The oscillating electrons in
the antenna will produce an electromagnetic wave of the same
frequency around the antenna and it travels away from the
antenna at a speed comparable to the speed of light in air.
This is how a transmitting antenna works.
A receiving antenna is the complement of the above. An
electromagnetic wave traveling through space (or air) of the
right frequency for the dimensions of the antenna causes the
charges in the antenna to oscillate in an organized fashion in
the antenna and an alternating voltage of the same frequency
will appear across a device (commonly called a receiver) that
is hooked up to it.
Whether it's a receiving or a transmitting antenna, resonance
is taking place and, in order for resonance to occur, the
electrons must be moving in a highly organized fashion.
2. What is taking place when a filament is producing visible light?
Before a light bulb is turned on, the atoms of it's filament are
all vibrating about in random fashion with an average kinetic
energy based on the temperature of the room. When you apply a
voltage to the filament, a current of electrons flows through
it, bumping into the atoms, transferring energy to them, and,
as a result, increasing their energy. This increase in the
random kinetic energy of the atoms means an increase in the
temperature of the filament.
At around 800° C, the energy is sufficient to free electrons
to a level where, when they randomly drop back down to their
lower energies, they give off visible light. Because this is
the minimum amount of energy needed for producing light, you
get light of the lowest frequency - the one we associate with
the color red.
As you increase the flow of charge, you generate more heat and
you get more of the colors of the rainbow - orange, yellow,
etc.. Eventually, they all coalesce to form what we call white
light.
In all of this, the primary source of energy leading to the
production of light is heat and throughout this process the
result is produced by electrons moving about in a highly
UNorganized, random fashion.
One can conclude from all of the above that, in coming up with the
filament, Edison did NOT invent a "visible light transmitting antenna".
Question A:
What exactly is a light antenna?
Compared to radio, tv, and microwaves, the frequencies of light waves are
very high and their corresponding wavelengths very small - from around 400
nm to 700 nm. So the light antenna needed to respond to light waves must be
a very small.
It turns out that carbon nanotubes fill the bill. They are electrical
conductors shaped like a wire and can be produced in very short lengths in
large quantities. Having a large number was important to the discovery
since the signal generated by a single nanotube wouldn't amount to much.
Using an array of randomly positioned nanotubes (think of gathering up a
large number of toothpicks that have been spilled out on the table so that,
while all are parallel to one another, they aren't neatly aligned side by
side between two parallel lines) , Yang Wang et.al. were able to detect a
change in the electrical properties of the array everytime a beam of light
of the right frequency fell on the array at the proper angle. As Paul
pointed out, when they then rotated the array 90° relative to the beam of
light the signal disappeared.
It was the combination of response of the array of nanotubes, whose lengths
were a multiple of 1/2 the wavelength of the beam of light that fell on it,
and the subsequent lack of response when the array was rotated through 90°
that established that the carbon nanotubes were acting in a way similar to
the way a radio antenna behaves with regards to radio waves. Since the
nanotubes were responding to light, they were called light antennas.
Regarding question C, "... are we talking about some kind of coherent,
modulated visible light?":
The only constraint on the incident beam of light was that it be the right
frequency for the length of nanotubes used. It need not be "coherent" or
"modulated".
Happy Holidays to all.
ron
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