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What colour is a mirror?

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Ian Dunn
303379.  Wed Mar 26, 2008 3:31 pm Reply with quote

To me, this is a rather confusing and complicated question.

The obvious answer would be to say a very shiny silver. However, if you try and look into a mirror to examine what sort of colour it is, you instead see the colour of yourself and anything else that is reflected in the mirror.

If you tried to examine it using a different method, like using another mirror or a camera, it still would not work because they would be caught in the reflection of the original mirror.

Logically, the only way you can tell what colour a mirror is, is to put it somewhere where there is no light and therefore no reflections, but then you cannot see the mirror.

Does anyone know the answer to this question?

 
AndyMcH
303396.  Wed Mar 26, 2008 4:08 pm Reply with quote

A mirror does not have a colour. It just reflects the light that hits it.
A window has no colour of its own. It simply transmits the light that hits it from the other side.

Water doesnt have a colour. Neither does air. Not everything has a colour.

Hope this helps ! :)

 
djgordy
303405.  Wed Mar 26, 2008 4:24 pm Reply with quote

Ian Dunn wrote:

Logically, the only way you can tell what colour a mirror is, is to put it somewhere where there is no light and therefore no reflections, but then you cannot see the mirror.


Things only have colour because of the light that falls upon them and which wavelengths are absorbed and which are reflected. If something is in a place that is devoid of light, then it will have no colour. This is true of mirrors and little yellow plastic ducks.

 
Ian Dunn
303415.  Wed Mar 26, 2008 4:38 pm Reply with quote

TubewayAndy wrote:
A mirror does not have a colour. It just reflects the light that hits it.
A window has no colour of its own. It simply transmits the light that hits it from the other side.

Water doesnt have a colour. Neither does air. Not everything has a colour.

Hope this helps ! :)


Actually, water is slightly blue. See Series A.

 
Davini994
303417.  Wed Mar 26, 2008 4:42 pm Reply with quote

Depends on the mirror. White, but often purple.

Claude Bile wrote:
The colour of a mirror (or any object) is defined by it's reflection spectrum when illuminated by a white source (I personally think a green ball in the dark is still a green ball).

The reflection spectra of mirrors is actually a very important characteristic in optics, particularly laser optics, so the colour of a mirror is not all that complex a question.

For example, most camera lenses have antireflection coatings to reduce Fresnal reflections. Since these coatings are optimised for the yellow/green part of the spectrum, the lenses (which can be regarded as mirrors - not all mirrors are 100% reflecting) have a purplish tinge because it reflects small parts of the blue and red spectrum, but not the green. The mirror is thus regarded as being 'purple' though you would never find a physicist officially regard it as such, but the purple colour of the mirror is quite obvious.


http://www.physicsforums.com/archive/index.php/t-72493.html

 
Davini994
303418.  Wed Mar 26, 2008 4:43 pm Reply with quote

djgordy wrote:
If something is in a place that is devoid of light, then it will have no colour.

Surely colour is something an object possesses, whether there is light reflecting from it at a particular moment or not?

 
djgordy
303423.  Wed Mar 26, 2008 4:54 pm Reply with quote

Davini994 wrote:

Surely colour is something an object possesses, whether there is light reflecting from it at a particular moment or not?


Absolutely not. An object may possess properties which may give it a certain colour under certain condition but if you change those conditions then the object will appear to be a different colour.

It's a bit like mass and weight. An object on the Earth will have the same mass as the same object on the Moon. However, the weight of the object will be less on the Moon than it on the Earth because the Moon's gravity is weaker. The mass of the object is inherent but the weight of the object is contingent.

Suppose you take a sheet of white paper. If you look at it in normal daylight it will appear to be white. But if you take it indoors and look at it under a red light bulb, it will appear to be red. Obviously, we regard the kind of light we have on Earth as "natural" but if we lived on a planet that had a red sun, Krypton for instance, we would have an entirely different perception of what is natural. Although we may well develop super-powers.

 
Davini994
303426.  Wed Mar 26, 2008 5:00 pm Reply with quote

I disagree completely;)

Something can be blue but appear to be red under a red light. It's still blue. "What colour is an object?" is the same as question "what colour does it look in white light".

The answer to this question is invariant if it is currently under a red light, or in the dark.

For the analogy you've used to work, I guess you must consider the question to be equivalent to "what colour does it look to me right now".

This has all gone very Descartian very quickly;)

 
Sadurian Mike
303429.  Wed Mar 26, 2008 5:01 pm Reply with quote

I just looked in my mirror and it was Mike-coloured.

Aren't the backings of mirrors silvered?

 
djgordy
303439.  Wed Mar 26, 2008 5:12 pm Reply with quote

Davini994 wrote:
I disagree completely;)Something can be blue but appear to be red under a red light. It's still blue. "What colour is an object?" is the same as question "what colour does it look in white light".


Something seen under a red light wouldn't look blue so it couldn't be blue. The question "what colour is an object" is not the same as "what colour does it look in white light". "What colour is an object" is a question about what, if any, are the inherent properties of the object. "What colour does it look under white light" is a question about our perception of that object under certain narrow specified condition.

 
Mort
303474.  Wed Mar 26, 2008 5:51 pm Reply with quote

I quote the guy from the Physics forum who explains this all rather well

DaveC426913 wrote:

You could extend this to say what is the color of a piece of paper when in pitch black visibility, that would normally be white by day. Techinically the paper would be now black.

It's more than technically, it is how it is taught in colour theory.

The perceived colour of an object is determined by the properties of the light cast upon it.
The perceived colour of an object is influenced by the properties of the object.

Think about it. The range of colours that an object could have is rigidly bound by the range of light that illuminates it. FIRST.

Only then, can the properties of the object further determine - within that narrow range - a smaller subset of colours that the object will exhibit.


Blue light limits an object's possible colours to a blue range. A red ball will have to appear purple - or black.


P.S. "perceived colour" is redundant. Colour is - by definition - about perception. Objects do not have colour, they have ranges of frequency that they absorb or transmit. The red ball under the blue light absorbed all freqs of light that impinged on it, and transmited none. But this does not make the ball red.

 
Davini994
303496.  Wed Mar 26, 2008 6:09 pm Reply with quote

djgordy wrote:
Something seen under a red light wouldn't look blue so it couldn't be blue.

That's perfectly possible. In the real world colours are made up of different wavelengths, not just 690nm or whatever true blue is. So If all the incoming light is in a particular frequency range, it could indeed look red, if it's only reflecting the red bit of the spectrum that it would under white light (which looks blue).

djgordy wrote:
"What colour is an object" is a question about what, if any, are the inherent properties of the object.

And yet it's inherent properties change when it's in the dark, or basked in light from only part of the spectrum?

What you are arguing is that colour is not an intrinsic property of an object; i.e., if the light that is travelling from an orange is of wavelengths that together make purple, then the orange is indeed purple.

djgordy wrote:
"What colour does it look under white light" is a question about our perception of that object under certain narrow specified condition.


dj wrote:
Obviously, we regard the kind of light we have on Earth as "natural" but if we lived on a planet that had a red sun, Krypton for instance, we would have an entirely different perception of what is natural. Although we may well develop super-powers.

We don't live on krypton though, do we?

White light is more important than red for this question. It's a neutral colour, in that it's light with wavelengths from all parts of the human visible spectrum. (nor is our visible spectrum particularly a coincidence, it's where EM radiation of stuff outside of special places tends to be. e.g. the blueness of the sky is caused by the atmosphere).

If we were Kryptonian, then colour would be defined in terms of 'Kryptonian white', i.e. an even mix of our Kryptonian visible spectrum. Krypton white may be a mix of green, yellow, red, xinquan and chobolottzl. Redder stars have the entire spectrum shifted to the red, so they gain some intensity at one end and lose at the other, I think. Or it might not be so simple.

Interesting discussion though, keep going. I don't mean to sound as arrogant and cocksure as I do, it just sort of comes out like that - as I'm sure you know by now.

This is very similar to a lot of the thoughts that Descartes had in general terms, although I can't find my books on him at the mo (may a pox be on my shelves).

 
Mort
303501.  Wed Mar 26, 2008 6:18 pm Reply with quote

And you've outlined precisely the reason why 'colour' cannot be an intrinsic property of an object.

We define 'colour' using the human visible spectrum, Kryptonians define 'colour' using the Kryptonian visible spectrum. We both observe the same object, but observe different colours. 'Colour' must (in part) be due to the spectrum of light it is observed under.

White is only 'more important' than red from a human-centric perspective. Science isn't human-centric (there's probably a better word for this), there are frequencies of light above and below what we can sense. Our spectrum of visible light is nothing 'special'.

 
Sadurian Mike
303510.  Wed Mar 26, 2008 6:28 pm Reply with quote

Mort wrote:
Kryptonians define 'colour' using the Kryptonian visible spectrum.

Not in my experience. The one I met didn't have a clue what I was talking about; he just shook his head and left Asda with his groceries.

 
djgordy
303517.  Wed Mar 26, 2008 6:41 pm Reply with quote

Davini994 wrote:


djgordy wrote:
"What colour is an object" is a question about what, if any, are the inherent properties of the object.

And yet it's inherent properties change when it's in the dark, or basked in light from only part of the spectrum?


No, because colour is not an inherent property. If we say "what colour is an object?" the question implies that the object has a colour. It doesn't. It only has properties which enable you to perceive colour under certain circumstances.

The object does, however, have inherent properties, such as mass so we can ask "what mass is the object" without specifying the conditions (although, granted, its mass will change at highly relativistic speeds).

 

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