Radio waves vs light waves

On Thu, 19 Feb 2004 13:17:36 +0100, Thomas Borchert
wrote:

Katherine,

Physicists call it "radiation pressure" and it's
measureable in a lab.


you can drive spaceships with it, though. There's a great scifi story
by Arthur C. Clarke about a solar yacht race using radiation pressure
from the sun on big "sails".

I've always wondered about the following:

Ok - so we all agree that a light puts out a small amount of force.
Newton tells us that an equal an opposite amount of force would be
directed in the opposite direction. F=ma.

So why can't we use high-intensity lights powered by a nuclear reactor
as a source of space propulsion? Is the F so small and the 'm' so
large that the 'a' would be miniscule? There's no wind resistance to
overcome in space, so you don't have to fight against that. I imagine
gravity would still be a factor though.

-Nathan

So why can't we use high-intensity lights powered by a nuclear reactor
as a source of space propulsion?


They can. But the F is small, so you need a lot of t to reach much of a v, as
long as you are far enough from a g.

Such systems have only been developed in science fiction however because we are
not yet doing enough interstellar travel, and people are uncomfortable with
nuclear reactors in space.

Jose

--
(for Email, make the obvious changes in my address)

Ya did really good...
Just one point... to effectively refract i.e., focus radio waves the
antenna can be as small as one half wavelength in dimension...
If I had a chalk board on here where I could scribble and wave my hands it
would become clear - taint clear unless I talk with my hands - It's the
time/phase delay due to C that allows a half wavelength structure to
effectively refract,
i.e. the arrows all add up to the shortest path - see R. Feinman, et. al.

Larger is better, however... Look at the dish at Arecibo for example..
http://www.rainforestsafari.com/observe.html
It is an efficient focuser of very weak, short wave length, radio waves
because it is many wavelengths across, gathering numerous wave fronts in
phase, and focusing them on the sampling probe...

Similarily, a camera lens that works efficiently at lower light levels will
be larger in diameter for a given focal length than one that doesn't work as
efficiently... i.e., an f:2.8 lens versus an f:1.4 lens..

denny
"Katherine" wrote
To noticably refract radio waves, I think you'd need something with a
thickness at least on the order of the wavelength of the waves. VHF
radio has wavelengths on the order of several meters.

Hope i did all that math right,

--Kath

"Dennis O'Connor" wrote in message
...
Larger is better, however... Look at the dish at Arecibo for example..
http://www.rainforestsafari.com/observe.html

Radio antennae such as that one use reflection, not refraction, to focus
radio waves. In fact, I'm not aware of a single refractor radio antenna.

Similarily, a camera lens that works efficiently at lower light levels
will
be larger in diameter for a given focal length than one that doesn't work
as
efficiently... i.e., an f:2.8 lens versus an f:1.4 lens..

Larger optical lenses are "faster" (i.e. lower f- number) because they
gather more light. It has nothing to do with wavelengths.

Pete

Pete old pal, too much caffeine?
Firstly, rf energy does refract as it passes through the near field of a
tuned element/antenna... If it did not, no radio antenna could work - just
think, no MTV, what a shame...
A good place to look for this without a lot of calculus is in Les Moxon's,
G6XN, "HF Antennas For All Locations" published by the RSGB... Review the
discussion of Super Gain Antennas for a two element array consisting of a
driven half wave dipole element and a closely coupled parasitic dipole
director... The incoming wave front interacts with the director as it passes
over it, and is bent/refracted, focusing it on the driven element raising
the gain of the antenna in exactly the same manner as a telescopic lens...
John Kraus,W8JK, also has good discussions on this in his tome - Antenna
Engineering Handbook - ...

And you are correct that Arecibo dish is mainly a reflector - which,
interestingly, is simply a special case of refraction - but I was using it
to make the point about gathering power increasing as the diameter of the
antenna in wavelengths, increases - and it is one antenna most non technical
folks are familiar with......

Have a look at Feinman's discussion on the relationship between % of
reflected waves and % of refracted waves as light passes through glass with
two plane parallel surfaces where the distance between the two surfaces is
varied in fractions of a wavelength.. Fascinating... Reflection/refraction
are two sides of the same coin...

And I don't know what to think about your last statement... But let me
point out that the faster lens 'gathers more light' because it has a larger
diameter in wavelengths, compared to the slower lens (assuming same focal
length for both)

cheers ... denny

"Peter Duniho" Radio antennae such as that one use reflection, not
refraction, to focus
radio waves. In fact, I'm not aware of a single refractor radio antenna.

Larger optical lenses are "faster" (i.e. lower f- number) because they
gather more light. It has nothing to do with wavelengths.

Pete

Peter Duniho wrote:
"Dennis O'Connor" wrote in message
...

Larger is better, however... Look at the dish at Arecibo for example..
http://www.rainforestsafari.com/observe.html


Radio antennae such as that one use reflection, not refraction, to focus
radio waves. In fact, I'm not aware of a single refractor radio antenna.


There are indeed refracting antennas. A common type is a dielectric
antenna in the form of a plastic "wedge" shape that protrudes, for
example, from the end of a waveguide. Various vehicular "speed radar"
detectors used antennas like this in years past. Microwave antennas
can also employ solid lenses, made of materials like polystyrene, and
for larger lenses that would be heavy, a matrix of plastic balls
with an overall lens shape serves the same purpose while saving weight.
A convenient material to experiment with for making you own dielectric
antennas is common paraffin wax.

Tom Pappano, PP-ASEL-IA

Can the lens in your eye focus radio waves?

No.

There are many reasons for this, the most fundamental is that EM radiation (the
general term for "light of any wavelength even if it's not 'visible' light")
(EM stands for electromagnetic) reacts differently to matter depending on the
frequency ("color"). Some colors are absorbed, some are reflected, some are
transmitted. This is what makes something green, or blue, or white. In fact,
white flowers often reflect differeing amounts of UV radiation, so bees can see
a difference. This remains true throughout the entire EM spectrum. Water
absorbs IR radiation but passes visible light. Glass absorbs UV and IR,
passing visible light. If you go to your dentist and get an X-ray, the device
they focus on you passes X-rays but does not pass visible light. And so on.

As for focusing, this involves refraction, which has to do with the difference
between the speed of light in a vacuum, and the speed of light in the substance
it is passing through. (think of the matter as interfering with the light
waves as they go by, slowing them down to some extent). The amount of
refraction (if any) has to do with the frequency of the light to begin with.
"Chromatic abberation" is a defect in lenses caused by the fact that different
colors are refracted different amounts (and is the reason prisms work in the
first place). So, a lens that is designed for one wavelength will not work as
well with other wavelengths.

In the case of visible light, the wavelengths are within a factor of two of
each other. (purple, the shortest, is only half as big as red, the longest).
Radio waves are MUCH longer wavelength. Some of them can be the size of a
football field, while others are only a few inches long. That's a pretty wide
variation, and is very different from the wavelength of light (which is very
VERY tiny - much smaller than a microbe). In fact, the waves sent out by the
VOR are bigger than your eye itself. Not much chance of focusing there!

Incidentally, the wavelength is inversely proportional to the frequency. The
speed of light is about 300,000,000 meters per second. So, if you have a
wavelength of 100 mHz (close to VOR frequency), that means that 100,000,000
waves are going by every second. So, one second's worth of those waves
(traveling at the speed of light) are spread out over 300,000,000 meters.
Divide the two... 300,000,000 / 100,000,000 gives you 3 meters, or almost ten
feet. ONE wave is ten feet long! Your eye isn't focusing that worth squat!


if light is supposed to be made up of both waves and particles...


It's not. It is neither a wave nor a particle... in fact the concept of wave
and particle don't really apply to things of this nature. However, light
BEHAVES as if it were a particle sometimes, and behaves like it were a wave
other times. It never does both at once, and which it does depends on what you
are trying to observe. (yes, it depends on what you look at! weird!)

...how much does it weigh?

It weighs nothing but it has energy. Energy and mass are the same, like water
and ice are the same. When you turn on a flashlight, the flashlight does get
lighter, and all that mass is turned into energy sent out as light. The
difference is not very much, but it is there. The atom bomb works this way
too... only by harnessing nuclear rather than chemical reactions, the amount of
energy released is considerably greater.

Light does have momentum. If you shine a light on something, there is a
recoil, and it does push the illuminated object away. It just doesn't do it
very strongly. Solar sails work on this principle (to my knowledge they have
not yet been demonstrated, but they are a serious contender for unmanned
interplanetary flight). Again, it's a VERY weak force, but it is there, and
even a weak force, applied for long enough, out in space where there isn't much
resistance, can get you moving quite fast eventually.

That should get you started thinking.

Jose




--
(for Email, make the obvious changes in my address)

That should get you started thinking.

A master of understatement you are, in addition to being an excellent
teacher.

Thanks!
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

"Jim" wrote in message
Ok, here's another weird one... if light is supposed to be made up of both
waves and particles, how much does it weigh?

It is my understanding that light is not "made" of waves or particles but
waves or particles can be used to describe their behavior depending on how
one wishes to observe them ("wave/particle duality", I think it's called).

Then again, if you observe the wave/particle, your observation changes the
outcome of the observation. This is called the "Ya Just Nevah Know" theory.

Weird stuff, that physics.

But here's another one that I got from a book a friend form here mailed to
me some time ago: You are inside a perfect sphere that has a mirror
surface. Your eyes are squarely (or is it"sperely") in the middle of the
sphere. What does the reflection look like?

--
Jim Fisher

But here's another one that I got from a book a friend form here mailed to
me some time ago: You are inside a perfect sphere that has a mirror
surface. Your eyes are squarely (or is it"sperely") in the middle of the
sphere. What does the reflection look like?


It looks like the sound of one hand clapping.

Mu

Jose

--
(for Email, make the obvious changes in my address)