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"And if you look inside a mirror, it becomes you-colored."
August 20, 2012 5:43 AM   Subscribe

What color is a mirror? [slyt] [via]
posted by quin (56 comments total) 25 users marked this as a favorite

 
That was cool! Like Beakman's World but for adults.
posted by shortyJBot at 5:50 AM on August 20, 2012


if you can get past the delivery, there's an interesting fact about why mirror tunnels look greener the farther "in" you look, and a shout-out to the Proto-Indo-European "bhleg".
posted by dubold at 5:54 AM on August 20, 2012


Waited for them to compare and contract with bronze mirrors and prismatic mirrors; was disappointed. Cheerful presentation, though.
posted by ceribus peribus at 6:03 AM on August 20, 2012


contrast
posted by ceribus peribus at 6:03 AM on August 20, 2012 [1 favorite]


I learned something!
posted by The Whelk at 6:32 AM on August 20, 2012


Couldn't pay attention. Distracted by thumbnail of robot butt.
posted by dgaicun at 6:33 AM on August 20, 2012 [7 favorites]


distrasted
posted by dgaicun at 6:34 AM on August 20, 2012 [2 favorites]


Mirror-colored.
posted by Faint of Butt at 6:38 AM on August 20, 2012 [1 favorite]


SPOILER
. . .
. . .
. . .
. . .


slightly green (-ish)
posted by exlotuseater at 6:43 AM on August 20, 2012 [1 favorite]


Hue might say...
posted by Smart Dalek at 6:46 AM on August 20, 2012 [1 favorite]


Did anyone else think the segue into the color of eyes was weird?
posted by oddman at 6:53 AM on August 20, 2012 [1 favorite]


slightly green (-ish)

You can tell this just by looking at a mirror edge-on.
posted by shakespeherian at 6:57 AM on August 20, 2012 [2 favorites]


I worked in a metal plating shop one summer and was amazed to see the variation in colors of plating. Nickle is distinctly yellower than straight chrome, which is blueish by comparison. We'd strike the base metal (usually steel) with nickle, which allows a better chrome plate, and you could see places where the chrome didn't "throw" over the nickle. When seen side by side, the difference is really obvious. And silver looks whitish compared to either chrome or nickle- or more accurately, silver looks more neutral. Also, front-surface mirrors will show a lot less of the color-shift in a mirror tunnel. Most glass is intrinsically green and for back-surface mirrors, when set up to make the tunnel effect, the light has to pass through two thicknesses on each reflection. The light doesn't pass through any glass when you use front-surface mirrors.
posted by Phyllis Harmonic at 7:00 AM on August 20, 2012 [10 favorites]


Too much bare chest.
posted by Nomyte at 7:16 AM on August 20, 2012


shakespeherian: "slightly green (-ish)

You can tell this just by looking at a mirror edge-on.
"

Then isn't that the color of the glass? If so, the video was wrong. And we still don't know what color the mirror is.
posted by Splunge at 7:19 AM on August 20, 2012 [2 favorites]


> a shout-out to the Proto-Indo-European "bhleg".

Alas, the OED doesn't agree: "further etymology uncertain; on formal grounds the word [black] could be from a base related to the Germanic bases of blank [...], but since this would give an expected meaning ‘shining, white’ there is an obvious semantic difficulty; many have sought to resolve this by hypothesizing that the word meaning ‘black’ originated as a past participle (with the meaning ‘burnt, blackened’) of a verb meaning ‘to burn (brightly)’ derived from this base..."

Aside from that, great video—thanks for the post!
posted by languagehat at 7:22 AM on August 20, 2012 [1 favorite]


Then isn't that the color of the glass? If so, the video was wrong. And we still don't know what color the mirror is.

You may be right, but I am too dumb about science to tell, TBH.
posted by shakespeherian at 7:23 AM on August 20, 2012 [1 favorite]


His diagrams show light reflecting off the top surface of the mirror, but most mirrors are coated on the back, so the light passes through the glass on the way in and on the way out. So yeah, you're really seeing the color of the glass, which is like the color of tropical water. The outside mirrors on my old Mercedes are coated on the outside, so you don't get a double reflection, which is nice except when you go to wash the mirrors, which scratch much more easily than conventional mirrors.
posted by weapons-grade pandemonium at 7:27 AM on August 20, 2012


Phyllis Harmonic: Most glass is intrinsically green

Things I Learned From Dwarf Fortress, Volume VII.
posted by Malor at 7:30 AM on August 20, 2012


Yup, the glass is greenish. That's what you're seeing when you look from the side (or, if you are too clever to think of the obvious, by looking deep into autoreflections).

To figure out the color of the mirror itself, simply look at the color of the mirror coating when it isn't polished to a mirror finish. Silver and aluminum are common; both are "white" (silvery, but without noticeably hue) to the eye.
posted by IAmBroom at 7:32 AM on August 20, 2012 [1 favorite]


The outside mirrors on my old Mercedes are coated on the outside

Whoa, seriously? I thought front-silvered mirrors were reserved for delicate optical systems.
posted by scose at 7:45 AM on August 20, 2012


Aren't those car mirrors usually plastic-coated or something?

No I do not have a car, why do you ask
posted by shakespeherian at 7:48 AM on August 20, 2012


Silver isn't really a color. It's a particular arrangement of grey, black, and white, as well as other colors in the area. That is why it's no big deal to re-create it using conventional paints or the LEDs in your monitor. You don't need "silver" paint.
posted by weapons-grade pandemonium at 7:52 AM on August 20, 2012 [2 favorites]


The older Mercedes mirrors are called "first surface mirrors". Dunno if they make them like that any more.
posted by weapons-grade pandemonium at 7:56 AM on August 20, 2012


I'm wondering when my monitor is going to get a "reflection" channel in addition to red, green, and blue.
posted by RobotVoodooPower at 8:01 AM on August 20, 2012


Here is a slightly long-winded explanation of how your headlight-dimming rear-view mirror works. The mirror is wedge-shaped, so at night you are seeing only the small percentage of light that is reflected off the front surface of the glass, instead of the mirror backing. So simple. I wish I had invented this.
posted by weapons-grade pandemonium at 8:05 AM on August 20, 2012 [1 favorite]


Did anyone else think the segue into the color of eyes was weird?

I'm guessing "Hyperactive Chain of Vignettes Leaping from Ridiculous Color Puns to How Mirrors Work to the Etymology of Black to Comparing Your Eyes to Outer Space and Also, Hey Here's Some Found Footage of Some Guy Running into a Wall, Brought to You by Pixy Stix for Adults: YouTube Edition" was probably too long a title.

Which is not to say I didn't find it entertaining and informative, but that was a crazily action-packed five minutes for a science video.
posted by psoas at 8:07 AM on August 20, 2012


Diffuse black with white on the specular channel.
posted by Uncle Ira at 8:11 AM on August 20, 2012


The segue was more or less about diffusion vs. specularity, I think.

I worked in the glass/mirror industry years ago. I nth the opinion that the greenish tint is almost entirely from the glass.

Modern plate-glass, back-silvered mirrors are an incredible feat of engineering, though. The basic process makes them pretty damn accurate. Float glass is made by "floating" the liquid glass on a bed of molten metal; that makes the glass surfaces almost perfectly parallel on both sides. Silver is applied on the back by "sputtering" - the glass is given an electrical charge and an aerosol spray of silver is given the opposite charge. The silver can be applied equally, extremely smoothly, and only microns thick by electrical attraction. Tin is applied behind that to add strength, and a protective paint is then applied to prevent oxidation.

Simple, but highly accurate.
posted by Benny Andajetz at 8:20 AM on August 20, 2012 [6 favorites]


You can tell this just by looking at a mirror edge-on."

Then isn't that the color of the glass? If so, the video was wrong.


Correct on both counts!

The glass is green. When you see the reflection of a (glass) mirror reflecting a (glass) mirror reflecting a (glass) mirror, and so on, you're effectively looking through a whole lot of stacked glass, and can see the colour that way.

It's exactly like how water is blue, but you can only really tell if it's fairly deep.
posted by Sys Rq at 8:29 AM on August 20, 2012


Aluminium reflection spectra peaks at about 0.4 micron in the visible region. So it's kind of a very light blue colour.
posted by public at 9:34 AM on August 20, 2012


"The older Mercedes mirrors are called "first surface mirrors". Dunno if they make them like that any more."

My VW has them- I think a lot of cars do these days.

BTW, you can check to see if a mirror is front-surfaced by holding the edge of your fingernail against the surface. If the real and virtual image are in contact, it's a front-surface mirror. If there's a separation of the images, it's a back-surface mirror.

Tiny derail:
For a long time, scientists were baffled by the fact that you can see the sky's reflection from the surface of a pond, and at the same time, see the bottom. Some of the photons reflect from the surface but others go through the surface to reflect off the bottom. How does any given photon "decide" to reflect off the surface or reflect off the bottom? The long answer is in Feynman's wonderful little book, QED, but basically, it comes down to probabilities and the strong likelihood that ~3% of the photons take paths that have them reflect from the surface.
posted by Phyllis Harmonic at 9:35 AM on August 20, 2012 [1 favorite]


The greenish tint in glass is caused by iron impurities. Similarly, other elements are added on purpose to create colored glass (cobalt blue for example).

The addition of lead to glass increases the clarity and refractive index creating what is commonly known as crystal.

Optical glass for use in camera lenses and scientific equipment is made to higher tolerances (lower impurity) to prevent color cast in the glass.

A mirror made with leaded glass crystal would not have a green cast.. but with the higher refractive index the colors might seperate causing a rainbow appearance of light.
posted by j03 at 9:36 AM on August 20, 2012 [3 favorites]


Man, I once tried some bhleg at a Proto-Indo-European restaurant. Never again.

And I wish they would have explored the answer "depends on the illumination" a bit more. Because there's a great example of it in the news these days. Pictures that Curiosity is sending back from Mars are sometimes "white balanced". Apparently the ambient color on the surface of Mars has a different enough spectrum from that on the surface of the Earth that they modify the images to show what they would look like if they were on the surface of the Earth, a.k.a. white balancing. I think it help the geologist recognize rocks, as they are all used seeing them with Earth's illumination. But it also points out that part of the reason Martian rocks look red is that Martian light is red.
posted by benito.strauss at 9:42 AM on August 20, 2012 [1 favorite]


Silver is applied on the back by "sputtering" - the glass is given an electrical charge and an aerosol spray of silver is given the opposite charge.

So I'm not familiar with all aspects of sputtering, but that's not how sputtering usually works. In my experience you create a gas plasma (argon usually) by some means and accelerate the gas ions to a target of the metal you want to deposit. The gas atoms crashing into the target causes the target material to be ejected and deposited on the desired substrate.

There's a Wikipedia article that seems to confirm my recollections here.
posted by All Out of Lulz at 9:42 AM on August 20, 2012 [1 favorite]


So I'm not familiar with all aspects of sputtering, but that's not how sputtering usually works.

I stand corrected. I sold glass commercially; I didn't make it. This was how it was explained to me at various plant tours. (I watched them do it, though.)
posted by Benny Andajetz at 9:55 AM on August 20, 2012


I bet mantis shrimp know the answer.
posted by orme at 9:57 AM on August 20, 2012 [1 favorite]


I get the feeling that most of the facts stated in this video are from the latest dubiously-sourced "crazy facts you didn't know"-type email.
posted by mrnutty at 10:13 AM on August 20, 2012


How mirrors are made.
posted by monospace at 11:40 AM on August 20, 2012 [2 favorites]


Wait a minute, Beakman's World isn't for adults?!?
posted by humboldt32 at 11:59 AM on August 20, 2012 [1 favorite]


j03:
A mirror made with leaded glass crystal would not have a green cast.. but with the higher refractive index the colors might seperate causing a rainbow appearance of light.
The rainbow is caused by the variance of the index of refraction over the visible spectrum. High or low index values make no difference. (Optical Engineer, BTW... pretty damned sure about this.)
posted by IAmBroom at 12:05 PM on August 20, 2012


public: Aluminium reflection spectra peaks at about 0.4 micron in the visible region. So it's kind of a very light blue colour.
"Very light blue colour" = not discernible by humans, for the most part, so: white.

We're talking photometrics, not photonics. If the human eye response is identical to white, it is white.
posted by IAmBroom at 12:09 PM on August 20, 2012


Silver is applied on the back by "sputtering"

And it would have to be RF sputtering too, to coat an insulator.

But you can also just electrolessly deposit silver through chemical magic. It's how people have made reflective telescopes for centuries.
posted by Chekhovian at 12:24 PM on August 20, 2012


Then isn't that the color of the glass? If so, the video was wrong. And we still don't know what color the mirror is.

A back-silvered mirror is made out of glass. So if the glass is green, the mirror is green.

Does anyone know why they use glass to make front-silvered mirrors? I would think anything flat would work fine, and there's lots of things that are stronger than glass.
posted by aubilenon at 2:14 PM on August 20, 2012


aubilenon, they use glass because: it's easy to make very flat (no grain), very rigid, and has a relatively low thermal expansion (2-5 e-6/deg F, versus 8-10 e-6 for steel).

Plus, two kinds of glass/ceramic hybrids - pyrex and ULE - can have incredibly low TCEs. ULE is something like 0.1 +/-0.3, so the TCE can actually be essentially zero on some lucky lots.

Most of this doesn't matter for household applications, where mirrors are also put on acrylic sheets as well. But if you're building a 1-m or bigger telescope, you bet it matters.
posted by IAmBroom at 2:28 PM on August 20, 2012


Great thread!

What would a person (vs an invisible point source - which would be undifferentiated general light, right?) see if they were in the middle of a perfect sphere about 5 meters in diameter with reflective inner walls? Given a light source, of course.
posted by porpoise at 7:07 PM on August 20, 2012


They would see the very depths of their soul. And it would drive them irreversibly insane.
posted by benito.strauss at 7:51 PM on August 20, 2012


Here's one that I heard and I'm not sure what the answer would be.

You are floating in a pure vacuum. And a pure mirror surrounds you. A perfect mirror. You have a flashlight in your hands. You are floating in the center of a pure mirror. No air.

You turn on the flashlight.

What do you see?
posted by Splunge at 9:31 PM on August 20, 2012


On lack of preview what porpoise said. But with a person in the sphere.
posted by Splunge at 9:34 PM on August 20, 2012


This may have been mentioned before but the mirror is slightly green because of iron in the glass used to make the mirror. Wouldn't the solution then be to use low-iron, sometimes called Starfire, glass to achieve a less green mirror? Would still have a bit of green in it, I'm sure, but significantly less than your average piece of float glass.

For a more perfect infinity tunnel!
posted by RobertFrost at 6:22 AM on August 21, 2012


They could use higher quality glass (and they do spend a great bit of time on the formulas; nickel occlusions, for example, can cause tempered glass panels to spontaneously burst), but it's just not needed for anything but fringe cases. The green tint isn't even noticed by most people in typical use. You have to "amplify" the effect with an infinity tunnel to even see it.

The big glass guys like Cardinal or Spectrum or PPG churn out miles of glass very day. Good is good enough for most cases.
posted by Benny Andajetz at 7:11 AM on August 21, 2012


What do you see?

You mean before you die from asphyxiation and exposure to vacuum?
posted by Dr Dracator at 8:00 AM on August 21, 2012


porpoise: What would a person (vs an invisible point source - which would be undifferentiated general light, right?) see if they were in the middle of a perfect sphere about 5 meters in diameter with reflective inner walls? Given a light source, of course.

Splunge : You are floating in a pure vacuum. And a pure mirror surrounds you. A perfect mirror. You have a flashlight in your hands. You are floating in the center of a pure mirror. No air.
You turn on the flashlight.
What do you see?


Great questions, you two!

The answer is: a horribly out-of-focus blur made up of the parts of your body in every direction. Imagine a photo printed with water-soluble ink, then immediate rained on. Now cover it in wax paper. Now bring it so close to your face that you can't even focus on it. That's what you'd see.

Unless... some part of your OUTSIDE surface happened to be at the exact center (focus) of the sphere, in which case roughly half or more of the sphere would be the color of that point on your surface. The rest of the sphere would only see the off-center, massively defocused rest of you, since you were blocking it's view of the point.
posted by IAmBroom at 3:59 PM on August 21, 2012 [2 favorites]


Mirrored Geometric Animals by Arran Gregory
posted by homunculus at 4:35 PM on August 21, 2012


My entire life I have wondered what walls facing each other / an inward-facing sphere made of one-way mirrors would look like. MY ENTIRE LIFE.
posted by nicebookrack at 9:05 PM on August 21, 2012 [1 favorite]


IAmBroom - yeah, that's kinda what I figured*, but I still wonder if some mad reclusive Lich class POVRay heirophant hasn't been able to demonstrate this for realz.

*depending on the cross-section of the body and the position of the lightsource, there would be an exponent of two pools of shadows. The radia might also be different depending on the wavelength of light. I can't intuit how minor differences in the speed of different wavelengths might do . . something?
posted by porpoise at 11:01 PM on August 22, 2012


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