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Thread: One Thing Which Always Troubled Me

  1. #1 One Thing Which Always Troubled Me 
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    Physics Class, High School, Sophomore Year, 1957, new teacher, Textbook Chapter "Radiation"

    Author of text stated, "X-Rays are much like visible light, but have shorter wavelength". "X-Rays pass through many common materials easily". "X-Rays react in similar ways to lenses as does visible light".

    I asked the young teacher, how the hell do lenses bend or diffract X-Rays if they pass unimpeded right through the material making up the lens?

    He did not know. I still do not. Perhaps this belongs in "Physics", perhaps right here is far enough.

    What do you think of that? jocular


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    Brassica oleracea Strange's Avatar
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    Light passes through the material making up a lens. If it didn't the lens would be opaque and wouldn't focus the light.

    But I don't think you can focus X-rays with lenses because the refractive index is too close to 1. I believe mirrors are used in, for example, X-ray microscopes.


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    [Phased] Fresnel lenses.
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    Cooking Something Good MacGyver1968's Avatar
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    Is that like beer googles, Dy?
    Fixin' shit that ain't broke.
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    Genius Duck Moderator Dywyddyr's Avatar
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    Close to, Mac. Wear them and you'd pick up ANY woman.
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    X-ray optics - Wikipedia, the free encyclopedia
    While lenses for visible light are made of transparent materials that can have a refractive index substantially larger than 1, for X-rays the index of refraction is slightly smaller than unity.[1] The principal methods to manipulate X-rays are therefore by reflection, diffraction and interference. Examples of applications include X-ray microscopes and X-ray telescopes. Refraction is the basis for the compound refractive lens, many small X-ray lenses in series that compensate by their number for the X-rays' minute index of refraction. The imaginary part of the refractive index, corresponding to absorption, can also be used to manipulate X-rays: one example is the pin-hole camera, which also works for visible light.

    Edit: Many x-ray lens uses zone plate diffraction, not refraction.
    http://en.wikipedia.org/wiki/Zone_plate
    Last edited by Harold14370; January 14th, 2013 at 10:17 PM.
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    Quote Originally Posted by Dywyddyr View Post
    Close to, Mac. Wear them and you'd pick up ANY woman.
    Or in Mac's case, men that he thought were women.
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    Quote Originally Posted by jocular View Post
    What do you think of that? jocular
    The numerical value of the speed of light in a material ( like glass ) is not the same as the one in vacuum. Therefore, at the boundary between the two materials, the propagation angle with respect to the boundary line changes.
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  10. #9  
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by jocular View Post
    What do you think of that? jocular
    The numerical value of the speed of light in a material ( like glass ) is not the same as the one in vacuum. Therefore, at the boundary between the two materials, the propagation angle with respect to the boundary line changes.
    "Light" meaning humanly visible radiation, but what about X-Rays? jocular
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  11. #10 Why the Thread? 
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    My point was, that information sources (textbook, in this case) sometimes have incorrect information, placed therein either by oversight, or ignorance of fact. So what? Utilization thereof is a gamble, yes?

    What if: The text suggested Cobalt 60, for example, were perfectly safe to handle?

    Ahhh, no point splitting hairs, dead horses do not arise.

    jocular

    Edit: What if my memory is faulty, as suggested by others in another forum, and 60 is the wrong isotope?
    Last edited by jocular; January 15th, 2013 at 01:43 PM. Reason: losing it
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    Quote Originally Posted by jocular View Post
    "Light" meaning humanly visible radiation, but what about X-Rays? jocular
    Both visible light and x-rays are just electromagnetic waves, albeit with different wave-lengths. Therefore the same principles apply to x-rays as do to visible light.
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by jocular View Post
    "Light" meaning humanly visible radiation, but what about X-Rays? jocular
    Both visible light and x-rays are just electromagnetic waves, albeit with different wave-lengths. Therefore the same principles apply to x-rays as do to visible light.
    Please, now, I am not being argumentative, but seeking I guess, "closure", as everyone now says nowadays, so,

    if the "same principles" apply to both visible light and higher-frequency radiation, just where do those principles begin to fail? For example, the higher the frequency, given visible light, the less the "glass" bends it, thus enabling visible separation of the colors present in that particular light being used. As the frequency gets higher, the lens affects the radiation less. So, energetic X-Rays, for example, may be "bent" by the same lens almost imperceptibly (we can't see 'em, anyway). Now, light energy is blocked by all sorts of common materials, slightly so by glass, more so by others, depending on their "transparency" to light, I suppose? Taken to the extreme, cosmic rays, so called, are extremely energetic, extremely high in frequency, and have been detected undiminished in the deepest mines of the Earth. How do you feel the "principles" relate to them, or, do they apply at all? jocular
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    If you followed the link in the Wikipedia article for compound refractive lens, you would find that the lens works, but due to the very little focusing power, it needed a whole row of lenses in series, and a long focal length. I would assume that this would be carried to even greater extremes in higher frequencies.
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    Quote Originally Posted by jocular View Post
    Please, now, I am not being argumentative, but seeking I guess, "closure", as everyone now says nowadays, so,

    if the "same principles" apply to both visible light and higher-frequency radiation, just where do those principles begin to fail? For example, the higher the frequency, given visible light, the less the "glass" bends it, thus enabling visible separation of the colors present in that particular light being used. As the frequency gets higher, the lens affects the radiation less. So, energetic X-Rays, for example, may be "bent" by the same lens almost imperceptibly (we can't see 'em, anyway). Now, light energy is blocked by all sorts of common materials, slightly so by glass, more so by others, depending on their "transparency" to light, I suppose? Taken to the extreme, cosmic rays, so called, are extremely energetic, extremely high in frequency, and have been detected undiminished in the deepest mines of the Earth. How do you feel the "principles" relate to them, or, do they apply at all? jocular
    Certainly the same principles apply, but technologies and techniques might differ, as a result of accommodating practical factors such as the properties of available materials, and length scales, etc.

    Example: Notice that radio antennas and filters generally look quite different from optical lenses. Both manipulate EM energy, so why the difference? Length scales, mainly. Conventional radio design involves wavelengths that are somewhat large (say, centimeter-scales on up). The desire for compact equipment leads us to engineering practices that use structures of the same order in size as (and often smaller than) the waves themselves. That's the domain of conventional circuit design.

    Classical optical engineering involves structures that are many wavelengths in extent. Refractors, reflectors and diffractors then become the standard sorts of building block elements.

    At very short wavelengths, the interaction with material properties becomes particularly acute (and, for the most part, deleterious). X-rays pose a particularly large challenge. First, they are hard to generate efficiently. Second, most materials absorb x-rays very strongly. That often rules out conventional lenses. That's why refractive optics dominate the world of x-rays.

    But all that is engineering. That's not to be confused with the fundamental science (which applies to EM of any wavelength).
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    Regarding X-Ray production as being a not-easy process, way back when Sony Engineers were developing ever-more useful LEDs, the hierarchy quietly askled them to try to develop a laser emitting LED. Took them about 3 months, I heard. I suspect those guys were actually responsible for many of the break-throughs taken for granted today.


    If it is not yet available, (for all I know, it may be), they ought to entertain developing an X-Ray emitting LED! jocular
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  17. #16  
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    Quote Originally Posted by jocular View Post
    If it is not yet available, (for all I know, it may be), they ought to entertain developing an X-Ray emitting LED! jocular
    If you use an extremely generous definition of "x-ray," we do have such LEDs today, although we more conventionally call them ultraviolet LEDs. The record, I believe, is something like 200nm.

    The challenge in generating shorter wavelengths is in finding a suitable material. A good LED material not only needs to have the right bandgap (larger BG --> shorter wavelength), but that bandgap has to be direct (meaning that the momenta of the recombining holes and electrons have to be equal, since photons have tiny momentum). And you have to have an efficient way of getting electrons into the conduction band in the first place. Plus, you have to be able to grow the semiconductor (and high-BG semiconductors require high processing temperatures). After satisfying all those requirements, the material used for packaging has to be transparent at the desired emission wavelengths. Meeting all those criteria is hard.

    Oh, and as far as who gets credit for the recent set of LED breakthroughs, that would be Shuji Nakamura, formerly of Nichia Chemical (now at UC Santa Barbara). His is the story of a complete outsider entering a crowded field and beating all the competition. He gave us the first practical blue LEDs, which then enabled white LEDs, and a path toward ever higher efficiencies and a broader spectrum of wavelengths. He deserved his Nobel.
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    Quote Originally Posted by jocular View Post
    My point was, that information sources (textbook, in this case) sometimes have incorrect information, placed therein either by oversight, or ignorance of fact.
    You have omitted the category that accounts for a substantial number of instances of "errors" and is possibly the largest single category: simplification.

    Textbooks are targeted at persons learning about a topic through instruction. Unless they are at the most advanced stages of learning in that topic simplifciation is likely to be esential. Such simplification may take the form of generalisation, approximation, removal of exceptions, explanation by analogy, etc. Any and all of these would constitute errors if taken as literal and absolute. Failure to recognise this vital point will lead to confusion and disorientation.
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    John, I will take issue to the extent that when a physical process IS...........or, IS NOT........, there is nothing to be gained by using any in-between description of it, such as simplification, generalization, etc., and room for loss eventually by placing incorrrect information regarding the process into the learners' minds.

    jocular
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  20. #19  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by jocular View Post
    John, I will take issue to the extent that when a physical process IS...........or, IS NOT........, there is nothing to be gained by using any in-between description of it, such as simplification, generalization, etc., and room for loss eventually by placing incorrrect information regarding the process into the learners' minds.
    The trouble is, in science and the real world generally, there are very few things which have simple true/false answers.

    For example:

    Why is light (electromagnetic radiation) refracted by a transparent medium?
    Because of the index of refraction.

    But what causes the index of refraction?
    Well, the speed of light changes in a medium.

    Why does the speed of light change? I thought it was constant.
    Well, actually, the speed of light doesn't change. It travels at light speed between atoms but then is delayed by interacting with the electrons.

    How does the light know to carry on in the same direction after interacting with the electrons?
    Well, actually, it doesn't. So what we have to do is calculate the probabilities of each possible interaction and do a complex sum to calculate the most likely path. It just so happens that when we do that, it turns out to be the same as the classical wave model.

    Why?
    We don't know.
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    My Doctor remarked to me, "Steve, we really don't know why this shit lowers your blood pressure, when asked about the drugs. jocular
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    True, we do not know the full mechanics of how certain drugs function. We have holes in our knowledge. We are working hard on progressing our knowledge to plug in those gaps. Nothing new here.
    Last edited by pyoko; January 20th, 2013 at 03:09 AM. Reason: typo
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    I think this will fit better in physics. Not too often something gets moved out of the trash and into the main sections!
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    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

    "Gullibility kills" - Carl Sagan
    "All people know the same truth. Our lives consist of how we chose to distort it." - Harry Block
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    Because the index of refraction is so close to one for x-rays and gamma rays it is very expensive and generally impractical to make such a big instrument and related lenses for focusing X-rays. Instread devices have been invented for such focusing with limited success. Here's a link to one of them.

    Physicists Invent Lens for Focusing X-Rays - NYTimes.com
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    Quote Originally Posted by forrest noble View Post
    Because the index of refraction is so close to one for x-rays and gamma rays it is very expensive and generally impractical to make such a big instrument and related lenses for focusing X-rays. Instread devices have been invented for such focusing with limited success. Here's a link to one of them.

    Physicists Invent Lens for Focusing X-Rays - NYTimes.com
    Forrest -- that IS a refractive lens (and an old one, at that; the paper dates from 1996).
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    Quote Originally Posted by tk421 View Post
    Quote Originally Posted by forrest noble View Post
    Because the index of refraction is so close to one for x-rays and gamma rays it is very expensive and generally impractical to make such a big instrument and related lenses for focusing X-rays. Instread devices have been invented for such focusing with limited success. Here's a link to one of them.

    Physicists Invent Lens for Focusing X-Rays - NYTimes.com
    Forrest -- that IS a refractive lens (and an old one, at that; the paper dates from 1996).
    Yes, I expect there now could be more advanced models and methods than this given 16 more years of potential development
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  27. #26  
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    Quote Originally Posted by jocular View Post
    if the "same principles" apply to both visible light and higher-frequency radiation, just where do those principles begin to fail? For example, the higher the frequency, given visible light, the less the "glass" bends it, thus enabling visible separation of the colors present in that particular light being used.
    That is due to something called "Chromatic Aberration." Different frequencies of light actually travel at different speeds through glass.

    Chromatic aberration - Wikipedia, the free encyclopedia


    I think where your teacher was confused when they said X-rays pass through material is ...... they don't pass through all materials, and even those which they do pass through they don't pass through perfectly or to the same degree.

    Different frequencies of light resonates with different materials. It's just like how sound waves of different frequencies resonate with different materials. A deep bass sound may cause your whole house to shake, while a high pitched scream might pass right through the wall without affecting it. Or bounce off the wall... etc.





    As the frequency gets higher, the lens affects the radiation less. So, energetic X-Rays, for example, may be "bent" by the same lens almost imperceptibly (we can't see 'em, anyway). Now, light energy is blocked by all sorts of common materials, slightly so by glass, more so by others, depending on their "transparency" to light, I suppose?

    Transparency to specific frequencies is what matters. If you put a red piece of glass in front of a light bulb you get red light. Why? Because all the other colors are either being reflected or absorbed by that glass. Only red is being allowed to pass through.


    Taken to the extreme, cosmic rays, so called, are extremely energetic, extremely high in frequency, and have been detected undiminished in the deepest mines of the Earth. How do you feel the "principles" relate to them, or, do they apply at all? jocular
    Cosmic Rays are not light. The reason they are called "rays" is because the name was given to them before anyone knew they weren't light.

    We now know they are (mostly) high energy protons. But they're called "cosmic rays" just out of tradition.
    Some clocks are only right twice a day, but they are still right when they are right.
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    Cosmic Rays are not light. The reason they are called "rays" is because the name was given to them before anyone knew they weren't light.

    We now know they are (mostly) high energy protons. But they're called "cosmic rays" just out of tradition.[/QUOTE]

    Fascinating! Particles with sufficient velocity to penetrate miles of rock? At such velocities they ought to have enormous mass, and almost indistinguishable size. Consult Fitzgerald-Lorentz length contraction, mass increase with velocity. Not schooled enough for this question: do particles lose intensity in a similar fashion to EM waves, that is inverse-square to distance?

    Astounding! Please let us know where this may be found in greater detail. After travelling millions of miles through space, how can particles (protons) still retain such high energy? jocular
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    Quote Originally Posted by jocular View Post
    Fascinating! Particles with sufficient velocity to penetrate miles of rock? At such velocities they ought to have enormous mass, and almost indistinguishable size. Consult Fitzgerald-Lorentz length contraction, mass increase with velocity. Not schooled enough for this question: do particles lose intensity in a similar fashion to EM waves, that is inverse-square to distance?
    Comparing EM waves to protons is going to mislead you. Instead, think about at EM wave as the field produced by a large collection of photons. Then understand that the inverse-square drop is simply due to the fact that the surface area of a sphere grows as the square of radius, so that the number density of photons drops as the radius squared. Individual photons are not losing energy.
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    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by jocular View Post
    At such velocities they ought to have enormous mass
    They do. They can have energies as high as 1018 eV, which is about a billion times the rest mass of a proton.

    After travelling millions of miles through space, how can particles (protons) still retain such high energy?
    Unless they collide with something, they will not lose energy. (The inverse square law would only apply to a spherically distributed flux of particles. not individual particles.)

    Particles with sufficient velocity to penetrate miles of rock?
    BTW. That's nothing. Neutrinos could travel through light years of solid lead without even noticing!
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