(Meta)Physical question abt mirrors


I understand that a white surface is one which reflects all/most of the light which strikes it. So why can’t I see my face in a white wall? Is it because the wall is rough? If so, if one could mill the wall down to an exquisite smoothness, would it transform into a mirror? Does this mean that the colour of a mirror is “white”?

I’m not sure if I can answer this question directly, but I’ll supply some information and perhaps someone else can use it to answer the question.

A mirror is a shiny object that reflects all light. duh. okay, the glass in front of the mirror is not what is reflecting the light. Glass is transparent. Light goes through it with minimal scattering.

An object that is opaque would not reflect an object if it was smoothed, because even microscopically, it’s still rough. You wouldn’t be able to smoothen it.

The color white can be produced by a combo of three different colors of the spectrum. Not all are necessary.

I’m too tired to think anymore. I’ll try to come up with more tomorrow (later today).

I don’t know who first said “everyone’s a critic,” but I think it’s a really stupid saying.

Well, duh. That’s why I said “to an exquisite degree of smoothness”. By which I mean, UNTIL IT WAS ABSOLUTELY PERFECTLY SMOOTH. This is a thought experiment at this point, but so what?

For that matter, why doesn’t a mirror itself have the same problem? Why don’t microscopic bumps in the coating cause it to scatter light?

Not sure what you mean here… If your three colours are “red, green, and blue”, then yes, all three colours certainly are necessary. If you take two complimentary colours like red and cyan, then you’re right, but cyan is just green and blue anyway, so same difference.



No. Just because an object is exquisitely smooth doesn’t mean that it reflects all light. It may still absorb some color.

I’ve always wondered this myself, but the last posts responces sure do lack any specificity and I find particularly unpersuasive. I hope to hear a bit more detail in the difference between White and reflective.

Pete, you don’t know what your talking about. There are only three colors in the spectrum, everything else is a combination of them

The facts expressed here belong to everybody, the opinions to me. The distinction is
yours to draw…

Omniscient; BAG

I’m not sure, but I think it has something to do with wavelengths.

No, on second thought, I think what we consider “white” is not really white.

No, maybe it’s the degree of reflectivity?

Ack, someone else answer this.

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matt_mcl asks:

Basically, yes. Prior to the perfection of the ability to produce large, flat pieces of plate glass (more infra), handmirrors were produced by milling small pieces of bronze or silver to exquisite smoothness; examples of these survive from Roman times, and perhaps earlier.

The wall cannot, practically, be milled to such an exquisite smoothness; bumps that are large in comparison to the wavelengths of light in question must be removed. This is posible with metal, but not with sheetrock, plaster-and-lath, etc. As a Gedankenexperiment (a German term meaning, “The grant proposal was rejected”), however, yes; if you could mill the wall exceeding fine, like the purported mills of the gods do, it would become a mirror.

As Pete notes, the glass provides no reflectivity; that’s done by the extremely thin coating of amalgam (that’s what they used to use, anyway; mirror technology may have advanced) on it. The glass provides a cheap, rigid, and transparent substrate for the amalgam. As it is transparent, not only must the surface (as with a larger chunk of metal) be smooth, but the glass as a whole must be free of distorting uneveness of density, refractivity, etc. (As an actual experiment, try a sheet of alumin(i)um foil, flat and crumpled.)

Not really. “Color” has no Ding an sich; it’s a construct of the human mind. “Frequency” or “wavelength” is what we really want to talk about, and, as Ishmael notes, a mirror, at least in the colloquial sense, does not have to reflect all wavelengths equally.

Omniscient opines:
Pete, you don’t know what your talking about. There are only three colors in the spectrum, everything else is a combination of them

This happens not to be entirely the case.

As noted above, there is no “color”; “wavelength” (frequency is inversely and exactly related to it) is the determining factor. It is a trivial task to show that there are a very large number of wavelengths in a spectrum (leaving aside the paradoxes of Fourier transforms in finite beams, we may say that any finite beam contains a finite number of phota; since each photon has a single, characteristic wavelength, there can only be a finite number of wavelengths in a finite beam. OTOH, this number is vastly larger than the number that the human eye can discriminate between).

The human eye and mind can be caused to reproduce the sensation caused by any given wavelength by the suitable combination of three monochromatic lights (and they needn’t be “red”, “green”, and “blue”; they can be any frequencies, provided that:
[list=a][li]they can invoke the appropriate reflexes in the cones, and[/li][li]their intensies are not such that they destroy those cones[/list=a]But that’s not to say that the addition of those three frequencies create, sonehow, that fourth frequency.[/li]

“Kings die, and leave their crowns to their sons. Shmuel HaKatan took all the treasures in the world, and went away.”

For what it’s worth, you don’t know what you’re talking about, either. The colors of the visible spectrum are: red orange yellow green blue indigo violet.

Didn’t you ever meet Mr. Roy G. Biv?

I admit I’m no expert, but according to Newton, who was an expert, any three different colors passing through a prism will appear white. As far as I know, no one has disproved this.

Thanks for sticking up for me, Nickrz, but I have to point out that there is no distinct indigo. It was fabricated to make the name Roy G. Bv pronounceable.

I still hold to my original assertion, that an opaque object can not be smoothened. In other words, “mill[ing] the wall down to an exquisite smoothness” is a moot point.
It’s like saying “if we got an infinite number of people to stand one on top of the other, couldn’t we eventually reach the sun?”

I don’t know who first said “everyone’s a critic,” but I think it’s a really stupid saying.

There are two types of light receptors in the human eye. Rods react to light & dark only and function better in low light situations. Cones react to different wavelengths of light. (Mnemonic - rods are shaped like conductor’s batons which are black. Cones are shaped like ice cream cones & ice cream is colorful. :slight_smile: ) There are three sub-types of cones - Red/Green, Green/Red, and Blue/Yellow

Cones react in approximately the following manner:
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| ***** ----- ~~~
| *** *** ---- — ~~ ~~
| ** *** ---- ~~~-- ~~
|* *** ---- ~~~ — ~~
|* ---- *** ~~~ — ~~
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Red Orange Yellow Green Blue Purple


Where the height of the curve represents the relative number of pulses (per time unit) the cone sensor sends to the brain. (Note this is not an exact graph. It’s been 15 years since I studied this.) The important thing to note is that the curves cross and they peak at Red, Green and Blue.

We perceive white(grey/black) when all sensors in the same proximity in the eye are outputting at the same level. Otherwise, we will see a color.

As Akatsukami noted, color is a function of perception, rather than an intrinsic trait of an object. Something which is perceived as white not only reflects all wavelengths of light, but diffuses the light so that different wavelengths are blended into one color. Mirrors, OTOH, do not diffuse the light, so the color you perceive is the color of the object being reflected.

I vote for none of the above. Spectrum is a physics term referring to the array of wavelengths which an energy source emits. While it is usually referring to the visible wavelengths, there are no discrete colors in the spectrum. It is rather a continous shift from red to purple.

What Nickrz is referring to is the color wheel with the painter’s primary & secondary colors (with indigo thrown in, as Pete pointed out for pronouncibility; also because indigo is the most common dye in the world.)

The three colors omniscient is referring to is the minimum number of colors you need to create any color. These three colors are red, green and blue for emitted light; red, yellow and blue for absorbed light. This disparity is something I contemplate often.

Interesting trivia - when Kodak was working on color film, they found that almost all colors can be created with red & green (for reasons obvious from the chart above). It took them awhile to realize that blue was needed to complete the spectrum.

Not Kodak, but Polaroid.

And the “Land experiment” went even further. Pictures taken with red and green filters can be reproduced with red and white (i.e., no filter).

But all of this, including conventional RGB systems, is just trickery to subvert the kludge that is the human eye/brain color-detecting system. There are an infinite number of colors in the theoretical spectrum.

John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams

Pete wrote:

“Opaque” refers to an object one can’t see through. I have on my desk a shiny steel plate, which makes a serviceable mirror. I await your proof that you have X-ray vision and can see through steel.

Thank you, zyada, for finally making the simple statement that answers the original question.

Does the idea of “color” have meaning when divorced from perception? (a tree falls in a forest…) I’d say that there are essentially three colors: corresponding to those frquencies at which the cones have their peak absorptions. Overall color perception is a mix of these stimulations. The frequency spectrum reflected by an object is a different matter, and usually (not always) occurs in a continuum.

Yes, AuraSeer, an opaque object, such as metal, makes for a good mirror, when flattened and polished. But in my opinion, it’s not opaque in that condition. An opaque object is one of which we can only see the surface, not anything beneath or on the other side. If you can see your reflection clearly and distinctly, then you can’t see the surface of the metal. It is not an opaque object anymore. Well, it is, but you’re not looking at the object; you’re looking at the reflection.

I don’t know who first said “everyone’s a critic,” but I think it’s a really stupid saying.

Well, color me blue. I’m talking about the colors of the rainbow for cripe sake, the visible spectrum. Look up the word “color” in any dictionary and then tell me there is only one “color” between infrared and ultraviolet. Physics term, my patootie.

John - thanks for the correction - I’m great at remembering concepts, but terrible at names. One of these days, I will learn to just not to use the name.

Nick - I did not mean to imply there was only one color. I was trying to say that each color blended into the next one. You’re right in that Roy G. Biv are the traditional colors of the rainbow.

We could probably get into some serious flaming regarding the number of colors. The answer will depend on what perspective you approach the problem from: physics, physiology, tradition, art, computer graphics, the Inter-Society Color Council, interior decorating…

For the record, this is how my dictionary defines color:

And now for what Cecil had to say on color names: http://www.straightdope.com/classics/a2_168b.html

Mastery is not perfection but a journey, and the true master must be willing to try and fail and try again