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fizziks


Norman Albers
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I am settling into a new home and a candle is lit on the table. It looks orange and bright. It is nearly dark outside, and I see two reflections in the double-pane windows. The first is true to color; the second is a bit smaller, but pale green in color. Why is this tweaky image happening?

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most window glass has a natural green cast (look at the edge of a sheet of glass), also the light from the candle is wide band, and colors of a lower frequency will pass through it quite easily whilst higher frequency colors are reflected better (greens, blues and upwards), and the light has to pass through 1`st sheet of glass twice, doubling the effect.

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I am not convinced it is this simple. Granted that glass has greenish tint, meaning that green frequencies are passed preferentially. However, if I look at this candle through two, or even through four panes, it will not look green! I think duh fizziks is a bit more subtle, having to do with reflection. Clearly the second image comes from the rear plate of glass because as you move around the two images move distinctly, and because the second one is a bit smaller. . . . . . . . . . . .After a rousing tankful on the chainsaw, it strikes me that multiple reflection must be involved. Six or eight panes would start to create the color filtering seen. This must involve a series addition of weaker and weaker multiple reflections.

Edited by Norman Albers
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Sounds like you're getting set up in your new home. Perhaps looking at the candle through the glass it is brighter and the greenish color is washed out, as color perception depends on the intensity of an object. The reflection would be dimmer and perhaps the color shows more easily? I think our vision shows colors best with an optimum intermediate brightness.

 

On the other hand I've seen unusual secondary transmission through windows that are offset and dimmed and perhaps colored, so perhaps there's some weirdness going on after all.

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How does frequency dispersion (relationship) show in reflection? We know the first pane does two absorptions but also is the reflection off the outer pane strongly filtered also? If the glass passes green but absorbs other frequencies, is the reflection preferentially green? I guess so, sort of obviously (?)_ . . . .Then again, maybe Dak has it. Looking in daylight, the white window frame shows a green echo reflection, and if I look sideways it is even purple. Honest! . . . . . . . . .Dak, is this the low-E coating? This place was built in 2003.

Edited by Norman Albers
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Kind of a long shot, but interference perhaps? Like the rainbow effect of a soap bubble or oil film in sunlight. The pane thickness is awefully large for that, and window glass isn't all great a plain, to form a uniform color. Does the color change as you change veiwing angle?

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How does frequency dispersion (relationship) show in reflection? We know the first pane does two absorptions but also is the reflection off the outer pane strongly filtered also? If the glass passes green but absorbs other frequencies, is the reflection preferentially green? I guess so, sort of obviously (?)_ . . . .Then again, maybe Dak has it. Looking in daylight, the white window frame shows a green echo reflection, and if I look sideways it is even purple. Honest! . . . . . . . . .Dak, is this the low-E coating? This place was built in 2003.

 

 

I have no idea. I was just going off of the fact that my spectacles have an anti-glare sheen on them, and they reflect green, whereas my older non-anti-glare one's reflected true-colour.

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Send the glass to me I'll chuck it threw a spectrometer and we can work out whether it's just a reflection effect from multiple planes or a transmission effect...

 

Is there any angular dependence (to both the light source and detector (your candle and eye))?

 

Reflections can be highly frequency dependent.

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One possibility is that there is an anti-reflection coating on one or more of the panes. These coatings are based on interference, and their color characteristics depend on wavelength and angle of incidence.

 

I've seen an unusual coating once that transmits yellow and reflects blue (or was it the other way around)?

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Low-E coating is tuned to reflect infrared, I think. It would be a quarter-wave thickness of the IR.

 

The quarter-wave thickness presents a reflection of a half-wave difference which cancels some of the surface reflection. I guess that at twice this frequency, which is tuned to some microwave value (1 or 2 microns?) the film is reinforcing. At three times frequency, again there's cancellation. At four times, reinforcement again. Thanks to all of you for checking in with ideas. KLAYNOS, if I ever chuck anything through one of these I'll send a sample!!!. . . .I feel confused. Don't we make low-E glass to reflect back IR energy? Does this mean it is half-wave thickness? . . . . On the trail, I Wiki on 'insulated glazing'. . .

Edited by Norman Albers
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  • 3 weeks later...

The salient detail I've picked up is the statement that one layer of glass (of two) is coated, on the inside. I figure it's the outer one in this case. I caught a reflection of the sun, and it too was bluish-green. Another is that the coating is more dense than the glass. No one says, half- or quarter-wave...

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Cheaper glass has more iron contaminants, which is the source of the greenish cast YT mentioned.

 

There will be a slight difference in reflectivity from the dispersion, since the reflectivity depends on the index disp_BK7.gif

 

and normal incidence reflectivity is [math](\frac{n_1-n_2}{n_1+n_2})^2[/math]

 

(Air has a similarly-shaped dispersion curve, but a smaller change in n, so to first order it's constant)

 

So blue reflects more than red (~.044 vs ~0.041 reflectivity for this, which is BK7 glass), but our eyes are more sensitive to green/yellow, whch may have something to do with it

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We're getting warmer! We know the reflections from the coating are greenish-blue, and we know it is designed to reflect infrared, out in summer and hold it in for winter. It's hard to see such a strong change given the small percent change in your curves, Swansont, but thanks for good data. . . I am leaning to my guess of half-wave, whole-wave layer thickness. If it was tuned to reflect IR at three times the wavelength of green, say 1.2 microns, then it would pass waves half that length, and again reflect those three times (one-third). So it would pass low red visible, say 6000 A., but reflect 4000. Plausible?

Edited by Norman Albers
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B-b-bingo! Here in Feynman Lectures, Vol. I, p. 30-7, a section on thin films. "...some of the light is reflected both on entering and leaving a refracting medium. Then, if we look at the reflection of a light source in a thin film, we see the sum of two waves; if the thicknesses are small enough, these two waves will produce an interference, either constructive or destructive, depending on the signs of the phases. It might be, for instance, that for red light, we get an enhanced reflection, but for blue light... destructive interfering reflection, so we see a bright red reflection. If we change the thickness..."

 

Here is a good industrial sort of reference: http://www.winrepco.com/Low-e%20Glass.htm THEY ARE SAYING, "METALLIC COATING", but I don't yet know how-so. Pure metals manifest an imaginary index...

Edited by Norman Albers
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