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Jayne's avatar

There's a target in my arrow; or rather, a pattern of alternating light and dark bands. Why?

Asked by Jayne (6751points) June 27th, 2010

I noticed while looking into the end of an aluminum arrow that there was a pattern of perhaps 6 concentric bands of bright yellow light (the color of the slightly translucent nock at the far end, through which the light presumably entered) alternating with black, fading to darkness towards the periphery. The bands were all of equal thickness, and remained circular and concentric if the arrow was tilted (although they became slightly blurry), ruling out any kind of interior markings. The interior seemed to have more of a matte than a mirror finish. My guess is that this is the result of wave interference, although I would have though this would require something close to a mirror finish. Anyways, I can’t find anything through google addressing this; any ideas?

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wundayatta's avatar

Reminds me of the way they make steel for swords. You take layers of high carbon and low carbon iron and pound them together, fold them over, and pound again, and do this over and over. Depending on the way you do the folding, you get patterns that sound similar to the ones you are describing.

However, you are talking about aluminum, which is a pretty weak metal, I think. Perhaps they have done something similar, with some other material to strengthen the aluminum enough to make it a useful arrow. Here’s a short piece about aluminum metallurgy. Looks like aluminum is alloyed with silicon, zinc, and magnesium, among other things. This picture (I hope the link works), shows an aluminum alloy dagger which has some iridescence.

This is a more technical document about heat treating aluminum to make it more corrosion resistant. In point number three, it says, “3. The method of claim 2 for producing a golden-yellow iridescent conversion coating used for salt spray corrosion protection of an aluminum surface, particularly for 7000 series aluminum, wherein the amount of concentrated nitric acid added tothe solution is approximately 8 cubic centimeters.” This sounds kind of like what you’re talking about.

Jayne's avatar

@wundayatta; the geometry of the ‘target’ shows that it can’t be an actual pattern on the interior surface of the arrow. If it were, the bands (if each band covered the same length along the arrow) would grow in apparent width towards the opening. Also, if the far end of the arrow were tilted down, the apparent size of the bottom side of each band would shrink and the top sides would widen, due to foreshortening. Neither is the case.

wundayatta's avatar

A picture is worth a thousand words. Good luck figuring it out.

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