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Thread: Question about AR

  1. #1
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    Confused Question about AR

    I'm curious if anyone can answer a question I have about AR.
    I read in a magazine that AR works by shortening the wavelength of reflected light so that it creates destructive interfearance(sp?) with the light coming in, thus negating the reflection.
    My question is this: if it negates the reflection by causing destructive interfearance, wouldn't that also negate some of the light coming in? I mean, if two waves interact with destructive, both waves are negated. So does AR really let more light in to the lens? Or does it just keep reflections off the surface?

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    Master OptiBoarder lensgrinder's Avatar
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    Because of destructive interference

    "As there is no net reflected energy (and energy must go somewhere), the light that would have been lost by reflcetion is acually added to the energy transmitterd by the lens." Mo Jalie, Opthalmic Lenses and Dispensing

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    That doesn't really make any sense to me. You mean that the two cancel each other out, and increase the amplitude of the waves coming in? or soemthing?

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    Master OptiBoarder lensgrinder's Avatar
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    What is being said is that since the light was not reflcected back, due to destructive interference, you still have the energy from the light wave which is transmitted through the lens.
    Just because you negated the reflection does not mean that the energy from the light stops, it must go somewher and that is transmitted through the lens.
    Example if you take a pebble and drop it in still water it creates a wave if you drop another pebble to negate those waves that you just created the waves are no longer there but the water is still moving.

    Hope this helps.

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    ATO Member HarryChiling's Avatar
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    Last edited by HarryChiling; 02-21-2007 at 04:46 AM.
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    Quote Originally Posted by EyeFitWell View Post
    That doesn't really make any sense to me. You mean that the two cancel each other out, and increase the amplitude of the waves coming in? or soemthing?
    Wow, I visit the "Ophthalmic Optics" forum for the first time in 6 months and find a question that someone with physics degree (like me) can answer!

    First start with an uncoated lens. A portion of the light that hits the lens surface gets reflected back.

    Now you add the AR coat. The coat is half a wavelength thick, so some light gets reflected from the AR coat surface, some gets reflected from the lens surface, and since these are a half-wavelength out of phase, these undergo destructive interference and cancel out.

    Now think about the light that gets reflected off the lens surface, then hits the AR coat surface and gets reflected back towards the lens. That light travels through the AR coat twice, so the distance it travels is a full wavelength. This then interferes constructively with the light that is passing right through the lens, thereby increasing its intensity.

    The above is essentially a mechanical description of how the principle of the conservation of energy works. Whenever you come up with a mechanism that decreases energy flow in one direction, that same mechanism must increase the flow in some other direction. Or else the energy must be dissipated as heat. It can never just "disappear".

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    opti-tipster harry a saake's Avatar
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    ar

    john your correct, that is why Ar coated lenses are more efficient, a clear lens loses apx 10 per cent of the light passing through, with ar the lens is now apx 99 percent efficient, thus one can see the advantage of wearing ar at night when there is the least amount of light, especially while you are driving and the average windshield is losing another apx 20 percent. There was a valid point made on the board one day that in some applications you may want to think about more light coming in

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    John,
    Fannin and Gosvenor take the same approach as you. My problem with that theory is that you have the light reflecting at the first surface of the coating AND interfering with the incoming light. I believe that Harry Chiling is on the money here. In fact, the best answer comes fromn Richard Feynman who won a Nobel prize in physics for his work on quantum electrodynamics (QED). Feynman actually addresses the problem in one of his books, titled QED. While we teach wave theory and destructive interference as the cause of 'anti-reflection', only quantum mechanics, in my view, can explain why transmittance is increased.

    Regards
    David

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    Master OptiBoarder Darryl Meister's Avatar
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    I usually describe the effect in terms of constructive interference, as John has. While the conservation of energy principle serves as a nice guideline, telling us that the energy must go somewhere, it certainly isn't an explanation as to how the light transmittance is increased. After all, the excess energy could simply be absorbed by the materials and converted into heat.

    That said, the fact that light seems to "know" that a portion is being reflected, while the remainder is being transmitted and reinforced by constructive interference, can only be explained by the magic of quantum mechanics. This is similar to the "strange" interaction seen between photons during double slit interference.
    Darryl J. Meister, ABOM

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    ATO Member HarryChiling's Avatar
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    Last edited by HarryChiling; 02-21-2007 at 04:46 AM.
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    HarryAsakke:
    Have things changed, are new figures out. I was always taught that lenses reflected light (of course this was glass) at rate of 4% per surface, making reflection (as least of light strikeing the surface @90 degrees, when it get up to 42% angle of incidence reflection becomes "total internal reflection) a total of 8%. Now you got it up to 5% per surface.

    I have always wondered of AR claims that one get 7 or 8% more light as in the latter case one would have 100% transmission which no lens has as such a lens would be totally invisible.

    Chip

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    ATO Member HarryChiling's Avatar
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    Last edited by HarryChiling; 02-21-2007 at 04:46 AM.
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    Master OptiBoarder Darryl Meister's Avatar
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    It is easier to explain interference in terms of wave theory, I think thats why it is taught that way for opticians.
    Certainly. Wave theory was the basis of John's original explanation (i.e., using constructive interference), and the one I prefer.
    Darryl J. Meister, ABOM

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    I agree that QED is the only way to "really" explain what happens with light propagation. I was afraid to say that here, since most people look at me like I have three heads when I say the words "quantum electrodynamics". :D I'm really surprised to even see it mentioned here.

    There are many other "funny" things light can do, like in the double-slit experiement where it acts like a wave when it passes the slits (i.e. showing a diffraction pattern with constructive/destructive interference) then acts like a particle when it hits the screen (i.e. makes a distinct "ping" in a localized place on the screen).

    I could talk lots more about QED "funny business", but I think it would be off-topic on a web site devoted to eyecare!
    Last edited by John Sheridan; 09-19-2006 at 11:54 AM.

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    OptiBoard Professional Kyle's Avatar
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    this thread makes me happy

    Can you imagine a world in which all opticians were this interested in quantum theory?

    What I really want to know is what PAL the rabbit wears...;)

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    Quote Originally Posted by HarryChiling View Post
    I would venture to say that quantum optics is a bit overkill for most opticians. It is easier to explain interference in terms of wave theory, I think thats why it is taught that way for opticians. I know that it would be a pain to try and teach some of the people I work with quantum theory as I don't totally understand every concept myself. Maybe when opticians become more educated.
    For the record, I'm in school for my license in one of the hardest states to get a license. So, although I might not be a quantum physics major, I'd wager that I know as much or more about this stuff than the average optician you've encountered. I debated for two years whether I wanted my optician's license or to get a degree in Phyics or Optical Science. My background is in Chemistry. So, I'm not a dunce. I finally decided that even if I don't stop there, a license is a good step (at least financially!;) ) and decided to go for it. In NC, even the ABO only covers two sections out the 9 sections on the test.
    Last edited by EyeFitWell; 01-30-2007 at 05:30 PM.

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    In fact, the single most frustrating part of my job is that I am a bright science-minded individual. I find myself working for and with people who don't know squat, and it's annoying. In fact, while enrolled in the 'apprenticeship' route to be eligable to sit for the exam, I asked my manager a question. This is a licensed optician. I asked her (and her boss, the owner-also licensed) if there was an equation to determine the necessary blank size if using a stock lens. We had wasted several pairs of stock polarized lenses that summer when big was hot. I had thought about it all night and decided there MUST be a way to catch that ahead of time. I came up with my own formula and asked my bosses. They BOTH told me to quit being a little miss smarty pants. Come to find out, the equation was correct and ON THE TEST!!!
    It's annoying when I want to know how stuff REALLY works, even if you go way over my head, I've already heard the dummed down stuff. You know what I mean?

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    ATO Member HarryChiling's Avatar
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    Last edited by HarryChiling; 02-21-2007 at 04:47 AM.
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    Ouch. People selling eyeglasses and managing optical stores who don't know how to determine blank size? When I started in optical, I knew how to do that before lunch on the first day. I HATE when people want to stifle curiousity and initiative on the job, or anywhere. Eyefitwell, I hope your desire to better yourself and keep learning in the art and science of opticianry is lifelong, and I hope when you become a manager and master optician (if you are'nt already) you foster the same appreciation for learning and initiative in your staff, and never forget the "little people" who rewarded your initiative with rude comments. they were probably afraid of their ignorance showing.

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    Old Optician to New OD Aarlan's Avatar
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    Quote Originally Posted by EyeFitWell View Post
    For the record, I'm in school for my license in the hardest state to get a license. .
    What are the licensing requirements in NC?

    AA

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    Master OptiBoarder lensgrinder's Avatar
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    Quote Originally Posted by Aarlan View Post
    What are the licensing requirements in NC?

    AA
    Follow this link

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    Master OptiBoarder ziggy's Avatar
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    Quote Originally Posted by EyeFitWell View Post
    In fact, the single most frustrating part of my job is that I am a bright science-minded individual. I find myself working for and with people who don't know squat, and it's annoying.
    If you work in the field very long you will come to understand this is the norm,,not the exception.
    Paul:cheers:

  23. #23
    What's up? drk's Avatar
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    I have a practical side question:

    Since reflectance is directly related to index of refraction, and since AR coatings are not 100% effective...

    ...what would be the order of difference in effectiveness of a theoretical given AR process between, say, a 1.7 index lens and a 1.5 index lens? Is the difference 1% reflectance or less? Or is it a greater effect?

    In other words, if you AR-coated a 1.5 and a 1.7 lens, and used a light meter to measure reflectance, would the 1.5 outdo the 1.7 by very much?

    Thanks.

    P.S. As to the wave-particle duality of light, I was taught that when dealing with light on a "macro" level, the wave theory explains phenomena quite well. It's only when dealing on very small scales that particle theory becomes important.

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    Hmm....DRK, that's a good question. I know that higher indexes reflect more light, making the use of AR all the more important.

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    Quote Originally Posted by drk View Post
    In other words, if you AR-coated a 1.5 and a 1.7 lens, and used a light meter to measure reflectance, would the 1.5 outdo the 1.7 by very much?

    My understanding is that the reflectance would be all but the same. Efficiency of the coating is determined by the correct selection of coating materials for refractive index, the thickness in which they are applied, and adherence to the previous layer. So a "correctly" applied AR coating would level the playing field among the various index lens materials.

    Hopefully, someone more up on this will correct me if I am wrong.
    Last edited by Fezz; 01-31-2007 at 11:17 AM. Reason: spelling issues

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