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03/04/2008 12:51:46 PM · #1 |
Glass is a "slow moving liquid," right? I'm sure everyone has heard of windows drooping and becoming thicker near the bottoms as the glass "runs" slowly down.
Could this happen to lens glass? |
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03/04/2008 01:08:14 PM · #2 |
Originally posted by Anti-Martyr:
Glass is a "slow moving liquid," right? I'm sure everyone has heard of windows drooping and becoming thicker near the bottoms as the glass "runs" slowly down.
Could this happen to lens glass? |
Theoretically yes. IF you leave the lens in the same spot for decades without moving it at all. Which is what windows tend to do.
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03/04/2008 01:12:25 PM · #3 |
From Wikipedia...
"The observation that old windows are often thicker at the bottom than at the top is often offered as supporting evidence for the view that glass flows over a matter of centuries. It is then assumed that the glass was once uniform, but has flowed to its new shape, which is a property of liquid. The likely source of this unfounded belief is that when panes of glass were commonly made by glassblowers, the technique used was to spin molten glass so as to create a round, mostly flat and even plate (the Crown glass process, described above). This plate was then cut to fit a window. The pieces were not, however, absolutely flat; the edges of the disk would be thicker because of centripetal force relaxation. When actually installed in a window frame, the glass would be placed thicker side down for the sake of stability and visual sparkle.[22] Occasionally such glass has been found thinner side down or on either side of the window's edge, as would be caused by carelessness at the time of installation."
And also: "If glass flows at a rate that allows changes to be seen with the naked eye after centuries, then the effect should be noticeable in antique telescopes. Any slight deformation in the antique telescopic lenses would lead to a dramatic decrease in optical performance, a phenomenon that is not observed."
(You may take or leave the source as you like...it was handy.) |
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03/04/2008 01:14:25 PM · #4 |
Ok, so I'm a materials engineer and I can tell you that the whole "glass flows" is not true. Simple answer, for all practical purposes, glass is a solid, what you hear about windows drooping and becoming thicker near the bottom is because of the way they were made originally.
More technical answer: Glass does not undergo a phase change when reaching the glass transition temperature (glass transition temperature = temperature at which something stops being a solid and start flowing. Polymers also have a glass transition temperature even though they are not glass, it's just a term). As a liquid cools, atoms start re-arranging to obtain the lowest energy configuration, which happens to be a long range order, which is called a crystal. Accompanying this long range order is a change in density (whenever you put something in order it takes up less space, for example crap on your desk). If you look at a specific density vs. temperature plot, you'll see a vertical line, which means there was a phase change. Glass does NOT have this vertical line because it is not creating a crystal (long range order or atoms) and technically, does not change phase. Therefore, glass is considered a supercooled liquid. For all practical purposes, it's a solid or if you want, a liquid that doesn't move.
So why are windows thicker at the bottom. Have you checked the windows in your house lately? they are not thicker at the bottom. The windows that are thicker at the bottom are those from really old churches/cathedrals. People say "oh it flows so slow you don't see it but after 400 years it's obvious the glass is flowing downwards". Well, again, that is not true. The way glass was made (usually, but not always) back in those days was by having a spinning disc and then the molten glass was poured in the middle, as the molten glass hits the spinning disc it was to move outwards (centripetal force). Molten glass is very viscous (thick) and thus will not make a perfectly flat disc. Now you have a disc that is thicker in the center. You proceed to cut it into rectangles for your window, and one end happens to be thicker, so where do you put that thicker end? YES! at the base so that it stands up more easily.
So, lens glass is safe, unless you melt it of course. |
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03/04/2008 01:18:30 PM · #5 |
Originally posted by keibo84:
So, lens glass is safe, unless you melt it of course. |
Just because I want to ask...how hot would my lens have to get before it melted? Could this happen in a standard kitchen oven? |
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03/04/2008 01:22:34 PM · #6 |
Originally posted by KaDi:
Originally posted by keibo84:
So, lens glass is safe, unless you melt it of course. |
Just because I want to ask...how hot would my lens have to get before it melted? Could this happen in a standard kitchen oven? |
I don't know how lens glass is made, but i'm assuming they put something in it to prevent it from expanding or contracting due to heat/cold. In which case it would be similar to borosilicate glass (pyrex ware), which melts at ...pretty freaking hot. So your regular kitchen oven probable won't damage the glass, but there's a whole lot of other stuff in the lens that will, so don't try it ;-) |
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03/04/2008 01:23:51 PM · #7 |
Originally posted by KaDi:
Originally posted by keibo84:
So, lens glass is safe, unless you melt it of course. |
Just because I want to ask...how hot would my lens have to get before it melted? Could this happen in a standard kitchen oven? |
There's not an insurance claim or anything like that hanging on this is there? ;-) *runs before he gets a thump*
Message edited by author 2008-03-04 14:54:09. |
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03/04/2008 01:25:40 PM · #8 |
Normal glass melts at roughly 1500 degrees, depending on the exact composition. I have no idea about the glass in lenses. Your self-cleaning oven gets to maybe 1000 degrees, so I doubt it would melt the glass, even in self-cleaning mode.
The plastic parts of the lens, however, would probably be toast, so to speak. |
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03/04/2008 01:31:14 PM · #9 |
No insurance claim hanging on it. I just thought it would be in keeping with the thread theme "I just thought of something..." Curiosity may have killed the cat but it's never done me much harm. ;)
Also must mention that I can appreciate an engineer who uses ballpark terminology such as "pretty freaking hot"...might just be the high-point of my day. Thanks! |
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03/04/2008 01:58:36 PM · #10 |
Originally posted by KaDi:
Just because I want to ask...how hot would my lens have to get before it melted? Could this happen in a standard kitchen oven? |
This post begs me to ask... Why would you want to put your lens in a standard kitchen oven? Was it bad lens that day? Did it hurt you first? Did it blur at the moment of exposure? Did it change the DOF without warning? Are you losing faith and love in your Canon? Would you feel better if you sent the kit-n-kaboodle to me? I am pretty sure I would feel better... and I would be very kind to your lenses... no untoward heat exposure, I promise! :)
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03/04/2008 02:32:15 PM · #11 |
I'd be willing to put the Kit lens into the oven and fire it up.... :-)
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03/04/2008 02:59:59 PM · #12 |
Originally posted by dwterry:
I'd be willing to put the Kit lens into the oven and fire it up.... :-) |
Or you could send it my way... My mom actually has a glass kiln that she uses for the specific purpose of melting glass for art purposes ;) |
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03/04/2008 03:00:06 PM · #13 |
I have no idea what the melting point of standard optical glass is, but I think it's safe to say that the oven temps would do far more damage to the plastics than to anything else in a typical lens.
BTW, the "glass is a liquid" story is not a total myth - there are properties that glass shares with a very, very viscous liquid, but the "flow times" would be measured in many millions of years, not centuries. |
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03/04/2008 03:05:57 PM · #14 |
Originally posted by strangeghost:
BTW, the "glass is a liquid" story is not a total myth - there are properties that glass shares with a very, very viscous liquid, but the "flow times" would be measured in many millions of years, not centuries. |
Cool. Another thing for my friend to wait for. While in college, he said he was going to sit at the bar until, through quantum mechanics, his beer jumped from one glass to another.
He didn't, though.
Liar.
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03/04/2008 04:12:42 PM · #15 |
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