It’s tough to produce interference effects between two independent light sources – as others have noted, you need to have two sources coherent, which has in the past meant that both rays have generally come from the same source, with the path length difference relatively short. Paul Dirac was convinced that you wouldn’t see interference between two light sources, and said so in his book on Quantum Mechanics.
Then in 1963 Mandel and another guy demonstrated that you could, in fact, get interference between two different light sources, and published the result in Natuyre (vol. 198, pp. 255-6). Lots of people have done it since, usually with lasers (Am. J. Phys. vol. 61 242-5 1993, for example)
I still don’t think it’s “easy”, though (I’ve never seen it myself). I don’t know that it’s been demonstrated with anything except highly coherent sources and under very strict circumstances, and (as been pointed out above) the sort of device the OP asks for would like it to work over a broad range of colors and with “natural”, low-coherence light. I seriously doubt that you’re ever going to get this to happen. Even for a limited viewing area and range with ambient light I doubt you’ll ever see it. Lippmann photography is a useful effect that uses a broad range of colors with natural ambient light, and it requires very special equipment and treatment because the ranges over which the interference effects occur is so damned small. In a way, it’s the closest analogue to what you’re asking about
You see, sound waves are easy to generate to produce coherent effects, and have a long coherence range, and can work over a broad spectrum of wavelengths – all exactly the opposite of light.
Technically speaking, you’ve never NOT seen it your yourself
Since your an optics kinda guy, I thought this might interest you. Back in the 60s, early 70s?, some astronomers used IIRC what was called intensity? interferometry to measure apparent size of the largest stars.
Optically, its nothing like you expect when you talk about interferometry. Nothing precise about it. They had big moveable “telescope” mirrors mounted on a railroad track. The mirrors were crap precision wise, just giant light buckets. At the focus were just photometers, no imaging was involved.
They did some kind of correlation thing with the photon noise to measure apparent angular size of the sources.
They wrote a short book on the story, how they did, and the results. When they proposed it, there were a fair number of optical experts that were sure it could not work even in theory, much less practice. The non math, non theory parts of the book make for an interesting read of “science drama”.
Its a neat little book, and if our modest library had it, it cant be too hard to dig up.
I’ve heard two things about the anti-reflective coating on my glasses: that there was a destructive interference set up by the reflection off the coating and then the lens itself, and that it let more light through. The second was from opticians, not usually my most trusted source for physics tutoring, plus they were trying to sell me the coating.
On the other hand, my dad walked in when I first got the coating on a pair of glasses, and said, “magnesium fluoride”. So it does seem to be the same stuff used on telescopes. Wiki backs up the destructive interference idea, Anti-reflective coating - Wikipedia. Why then do telescopes use it? Wiki mentions getting better contrast through not getting “stray light”, but I can’t picture this. I couldn’t explain why or how this works to a bright high school student of mine.
First, many optical surfaces need to be coated with something “tough” because they are “delicate”.
Second, when you make a simple coating that causes destructive interference at one wavelenght, it cause constructive interference at another.
So, for a telescope/optics system, you make the coating so that it causes more of the light with a certain wavelength you want to get through. Like say the wavelength of light your sensor is most sensitive to. And sometimes you design a coating to NOT pass a specific wavelength.
I was thinking in terms of a collimated point source. When the light is refracted through the prism, you get a linear band where the location is related to wavelength. The mirror is sloped with the wavelength of the light so that the path length varies by half a wavelength.
Of course you would need a very smooth mirror and very sensitive mechanics to get a few hundred nanometer slope in the mirror. Peizoelectrics can get that type of sensitivity in AFM and the mirror could be as simple as gold deposited on mica.
Any time you pass from a material with one index refraction to another with a different index of refraction you will get a reflection, as when you go from air to water, or from air to glass. The bigger the difference in index of refraction, the bigger the reflection coefficient.
Most glasses have a refractive index of about 1.5. If you put down a layer of material for which the thickness multiplied by the refractive index of that substance is a quarter of a wacelength, you will get destructive interference at normal incidence. (The ray travels twice through the material, so it travels a half a wavelength between the time it enters the layer and the time it leaves after being reflected) The ray that travels through the layer is half a wave out of phase with a ray that simply reflects from the surface, so the amplitdes add, and cancel out. Bingo – you have reduced the strength of the overall reflected beam by making a beam that bounces off the surface of the Mag Fluoride and the ray that passes into the mag fluoride and reflects from the glass underneath very nearl cancel out.
For the most efficient cancellation, you want the refractive index of your coating material to be about the square root of the initial medium (air, with an index of 1, for practical purposes) times the square root of the underlying glass (about 1.5, as i say). The refractive index should this be about 1.225. For Mag Fluoride it’s 1.38 in the middle of the spectrum, which is close enough. Mag Fluoride is also tough, which is worth using even if the refractive index isn’t a perfect match.
Of course, this will only work for one wavelength, but you choose the thickness so that the wavelength is close to the middle of the visible spectrum, and live with the reflections from other wavelengths.
This is about the easiest anti-reflection (AR) coating you can do. It only requires a single thickness of material. Typically, coating engineers design “multi0layer stacks” that use multiple coatings of different materials, usually alternating a high and low refractve index. The more layers you use, the better you can extinguish reflections and the broader the spectral region you can cover, or the sharper you can make the cutoff.
As for Stray Light, using such an AR coating will reduce this only because it reduces reflected light. Stray Light is generally handled by using apertures and special coatings on telescope walls, which has nothing directly to do with coating.
Such astronomy using synthetic apertures to increase the effective size of the telescope is a common technique that has long been used to measure, for instance, the diameters of distant stars:
Actually, one problem with prisms is that the dispersion is NOT linear. It’s usually pretty far from it (except of a VERY tiny wavelength range). Optics folks tend to prefer diffraction gratings, for which the dispersion rule is a lot simpler, and IS usually pretty close to linear over a larger range than for prisms.
Cal. I know that. I know that you know about that too.
The interferometry I was referring to WAS NOT phase based, at least in the commonly understood sense of the term. It used basically giant shaving mirrors on railroad tracks for pete’s sake. If it was that crude how COULD it posssibly be phase based when working in the visible/near visible range ?
IIRC it was** intensity **based. I could be loosing my mind, but I don’t think its something most people think of when they think of interferometry, particularly these days.
Of course I know this, but a curved mirror is also possible. You just won’t be able to use the deposited metal on mica trick for the curved mirror. Although you could place it on a flexible surfaced then bend it with piezoelectrics. Since your talking about nm it wouldn’t have to be very flexible. Aberrations are inevitable.
First off, Bwuhh? The stray light is handled with coatings with have nothing to do with coating.
Do you mean that the coatings on the walls are unrelated to those on the lens?
Second, is the AR coating is reducing stray light by reducing reflections inside the scope, that might be reflecting off the lens after having first passed through it?
“Stray Light” generally refers to unwanted light that’s rattling around inside your telescope or whatever – even light on the wavelengths you want that’s going where it shouldn’t is “stray light”. Most of this is handled, as I say, by coating the walls of your telescope with stuff to prevent it’s reflecting cleanly off, and making sure it gets absorbed by good blacking, or by using apertures to block it.
Coatings are generally used to prevent transmission of wavelengths you don’t want and maximize transmission of wavelengths you DO want. In addition, such coatings should be “clean” and free of scattering, which will contribute to Stray Light.
I’m not sure what sense you’re using Stray Light in.
Sorry – I assumed you were referring to the usual case. I had no idea from your post that you were referring to anything different. I don’t know what you are, in fact, referring to. Your description doesn’t coincide with anything I’m familiar with.
Sorry to you, too, but that seemed to be what you were saying. I’m at a loss to understand what you are talking about here.
Nothing to be sorry about. I was brushing over the obvious non-linearity to make the principles of the device understandable. You would probably have to have some sort of physics degree to figure out what curvatures were necessary to make it work, but nanometer adjustments are comonplace with AFM and nearly atomicly flat surfaces are created daily (also for AFM). I’m just saying that I think it might be plausible if difficult to do. I have no idea if there are other unforseen issues.
No need to be sorry about it. But that IS why I brought it up. Because when I first ran across it, my reaction was “no frackin way can that work, it aint interferometry as I have ever understood it”. But there was book on it. In the non-fiction astronomy section. With lots of math that at first glance appeared solid. Apparently funded by the NSF or something like that. With published results.
If you are really interested in this, I can browse the library next week and see if I can find it.
My point is that your previous point included this:
Maybe you meant that the wall coatings have nothing to do with AR coatings on the lens.
As for the other thing I said, I meant, "are AR coatings used to interfere with light that might have bounced back to the bottom of the lens, to eliminate it from reaching the sensor out of phase with the good light, or out of place?
To be blunt about it, the “special coating” you use on the inside of the telescope walls is black paint. Really good black paint, maybe, but it’s a fundamentally simpler problem than making something that doesn’t produce excessive reflections but still allows light to pass through it, like you need on the lenses.
Amatuer telescope makers like some baffles and certain kinds of felt. Or finely crushed walnut shells mixed in with the black paint.
Before the really fancy black paints, you know what made a good black surface if done right (if I understand correctly) ? A block of razor blades clamped together.
The telescopes you refer to did use phase (there is no such thing as intensity based interference). I don’t remember the name of the telescope you speak of, but basically it measured the coherence length of the light. While the light from a hot object, like a star, is completely uncorrelated (as it is the product of a very large number of atoms radiating independently); light gains coherence as it propagates through space. What you need to understand this is a good book on statistical optics. Wikipedia has nothing to say about it… Neither does hyperphysics or Wolfram’s physics site; I am really surprised by this.
Anyway, since I cannot find a site online, you have only my word for it. If we define a parameter, say alpha, as the separation of two apertures (mirrors for the case of your telescope) times the radius of an incoherent source divided by the distance to the source, then the mutual intenisity (and the visibility of the interference function) will fall as J[sub]1/sub/[symbol]a[/symbol] where J is a Bessel function of the first kind. By increasing the distance between mirrors it is easy to find the nulls of the Bessel function and thus determine the radius of the star (as long as you know the distance and have a good wavelength filter - the light needs to be relatively monochromatic). For the sun, the first null of the Bessel function occurs for a mirror separation of ~8 microns IIRC (this is the coherence length of the sun at 1 AU).
Regarding anti-reflection coatings, all of these rely on interference and the coherence length of the source. For incoherent sources like the sun or light scattered by objects, this length is very small so the coatings need to be very thin.
[quote=“L. G. Butts, Ph.D., post:38, topic:514146”]
The telescopes you refer to did use phase (there is no such thing as intensity based interference).
[QUOTE]
But like I said, phase not in the “normal” sense of the word when one thinks of interferometry. You mention measuring coherence length by measuring intensity. That is probably what I am recalling.
When I think of astromonical interferometry, I think of two telescopes collecting light. You bring that light together in one spot. You optically adjust the optical path differences so that you can control it to a fraction of wavelength of light. It all very optically precise and rather difficult to do.
Obviously YOU understand this set up that I recall. But rather crude optics and no high tech correction of optical path differences, much less not EVEN bringing the two light sources together (IIRC) is your grandfathers interferometry, not what most folks think of these days.
What I was remembering was the Michelson stellar interferometer while what you were remembering sounds like the more advanced Hanbury Brown-Twiss interferometer. Both make use of the complex degree of spatial coherence, but the HBT relies only on intensity measurements without interfering the beams. Anyway, good stuff…
On another note, I read from your reply that I might have been coming off as an asshole (what with the bolded I and you); if this is the case, sorry, wasn’t my intention…
