Stealth Technology
Re: Stealth Technology
Narayanan said :<BR> <BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR><BR>I am curious as to WHY corners scatter so much, and why the radius of curvature (1/sharpness) of the corner matters. <HR></BLOCKQUOTE><P>Scattering of electromagnetic waves behave differently when the geometric dimensions of the sactterers are smaller or larger than the wavelength. <P>For spherical scatterers larger than wavelength, most of the sacttering happens only in forward and backward directions. There is not much scatter in the sides. If scatterer size is much larger than the wavelength and if the surface is flat enough then phases of scattered waves add up to give specular reflections like mirrors. The waves get reflected towards one direction primarily. <P> But when scatterer size becomes smaller than the wavelength, so called "Rayleigh Scattering" becomes prominent. Rayleigh scattering is polarization dependent, and it can have a scattering pattern which is uniform in a plane (for a concentrated point scatterer it would look like a dipole antenna radiation pattern). In such a case scattered waves go towards many more directions rather than being concentrated towards certain directions.<P>I think this might have something to do with the problem with the edges. Edges will scatter more in a "Rayleigh" manner, throwing waves in many more directions (mostly perpendicular to the edge). Flat panels reflect waves as plane waves into one preferred direction. But edges may become sources of secondary cylindrical waves moving in all directions perpendicular to the edge. <P> <BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>In laser scattering, the actual phenomenon is supposed to go something like: <P> a) photon hits molecule.<BR> b) molecule's electron gets excited. <BR> c) molecule emits photon<BR> d) exiting photons interact with incident photons. <BR> e) interference phenomena result. <P> Does something like this occur with radar as well? <HR></BLOCKQUOTE><P>What you are describing is scattering of an em wave from one single molecule. Electronic energy levels in molecules are much higher in energy and involve visible photons or x-rays. Radio wave photons don't have enough energy to excite elctronic levels. Rotational energy levels involve micro-waves, but rotation is important only for gases where molecules are free to rotate. Vibrational levels involve infrared. But in a metal the waves can be scattered mainly from the free conduction band electrons. In an insulator they get scattered by induced dipole moments of atoms by displacing the elctron cloud with respect to the nucleus slightly.<P> For radar scattering it is probably unnecessary to think in terms of molecules. You can provide the geometry and optical properties of the scatterer (refractive index and absorption coefficient), and then solve for scattered field using classical electromagnetic field equations of Maxwell.<P>Interference is an important aspect of scattering. If scattering can be assumed to be happening from a sactterer consisting of many point sources then adding up effects of all these points sources effectively means adding them with their phases which means interference.<P>Interference is not really an interaction between photons. Photons don't interact with photons. They only interact with charged particles like electrons. Interference can be thought of a redistribution of energy without interaction. If you cross two light beams, they will interfere where they intersect (giving rise to fringes if they are coherent enough), but once they pass through the interference region they will look as unblemished as before.<P> <BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Is there charge accumulation? <HR></BLOCKQUOTE><BR>If charges accumulate on a conducting surface they like to migrate towards areas of high curvature. Sharp points and edges can cause dielectric breakdown of air much more easily than flat parts. I am not sure how much charge can be accumulated on aircraft wings, but it is definitely possible that while moving through an atmosphere that has some ionization, the wings can pick up some net charge. There might be some ionization due to drag too. <P>Radio waves by themselves won't cause any photoelectric effect to create charges on the wings, but they may react with preexisting charges. These preexisting charges would have migrated to the edges and sharp points. Radio waves will scatter from these edges and sharp points in the Rayleigh way but with some modifications. Right now I have no idea how much of an effect net charge on wings' edges can have on scattering.<P> <BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>In the IIT many decades ago, one of my fellow denizens tried to hang a 60-watt bulb and a 40-watt bulb next to each other to get interference and produce a 20-watt light.<HR></BLOCKQUOTE><P>Ordinary light bulbs don't produce coherent light, so you won't be able to see any interference. Also energy is not created or destroyed in an interference pattern, but is merely redistributed from dark regions to bright regions. Actually an interference pattern exists for the light bulbs for an incredibly short time span, but due to incoherence, this pattern changes rapidly into some other pattern. The net effect to eye, which averages over much longer time scales, is that there is no interference pattern. Filaments in the bulbs have no info about what the emitted photons are doing outside. Energy emitted has to be conserved. Therefore averaged intensity due to a 40W bulb and a 60W bulb would correspond to a 100W bulb even with interference. <P>Coherence is just a measure of how correlated a wave is with respect to some other point on its wavefront (spatial coherence), or from some point on the wave separated in time (temporal coherence). Lasers as you know are the most coherent sources we have. They are monochromatic, and all the photons move in the same direction with same phase relationships in tandem.<P>Ashok Kumar.<p>[This message has been edited by Ashok Kumar (edited 04-08-1999).]
Re: Stealth Technology
Ashok: <P>That was very nice, thanks! Worth many hours of searching textbooks and doing calculations. <P>I believe the radar wavelengths are of the order of millimeters (is that right? I see references to "millimeter-wave radar" ?) <P>The radius of curvature of a sharp edge is also of the order of a millimeter. So, the scattering is what is called "Mie scattering", where the diameter of the scatterer is the same order as the wavelength of the incident radiation. (Perhaps other radar is longer wavelength, sending the edges into the Rayleigh scattering regime as you said). So, it is possible to get intense scattered radiation in certain directions (within "lobes" of the scattering pattern). <P>So that's why edges may light up. Makes sense. <P>Regarding my fellow-denizen, I know. He skipped the Physics class where they taught about interference. Another friend learned the hard way why people needed sodium lamps, instead of just a bright light, to do interferometry in supersonic flows. These days, of course, we just use lasers. <P>A dire warning to anyone who still has the opportunity: DO NOT SLEEP in those Physics and Math and Chemistry classes just because you are in engineering! All of it comes back to bite you later in life. <P>
Re: Stealth Technology
Yogeshwar: <P>Regarding Vermiculite: <P>All I have heard of this is that when we dealt with a certain large aerospace company, the contract included a big stipulation that <P>'VERMICULITE SHALL NOT BE USED IN PACKING" any of the stuff sent to them. <P>I had to go around asking what this stuff was, and someone told me it was used to absorb water, and used in the pots for indoor plants. Perhaps it has some strange properties such as not passing X-rays, etc.
Re: Stealth Technology
<I>A dire warning to anyone who still has the opportunity: DO NOT SLEEP in those Physics<BR>and Math and Chemistry classes just because you are in engineering! All of it comes<BR>back to bite you later in life.</I><P>How true! How very, very true!<BR>
Re: Stealth Technology
Narayanan,<P>Yes, the general theory that describes scattering from an arbitrary sized spherical scatterer is Mie scattering theory. Rayleigh scattering is a large wavelength limit of Mie scattering.<P>Ashok Kumar.