White v Silver

[Buenchico]

Many years ago, when I was a student teacher, I had just finished a lesson about light when a 12-year-old girl asked this: "Sir, if a surface which which reflects all frequencies of light appears white, why is a mirror silver?"

30 years on, I'm still not sure exactly how I should have answered this! Any suggestions please?! (Remember that any answers should be concise and comprehensible to a 12-year-old of average intelligence!). BTW: I think that this was when I decided to teach Maths instead of Physics!

[Clanad]

Unfortunately, you, as well as the student, have assumed a connection between the two parts of the question that doesn't exist.  Firstly, the mirror without the silvering (aluminum oxide, mercury or potassium sodium tartrate, etc.) would only be a piece of glass with little, if any reflective properties.  Secondly, a mirror can reflect any color available from the spectrum that is visible to the human eye and, therefore, does not appear to be any color other than the color(s) it reflects... As proof of this hypothesis, a white object will appear white when reflected in a mirror, but any other object will appear to be it's true color, while the white object remains... well... white...

[Gef]

A mirror has a highly polished surface and so you get a true reflection. A piece of white paper, for example, reflects most of the light but since its surface is rough the light is scattered in all directions.

[Buenchico]

Responses to answers posted so far:

To Clanad:  Many thanks for your reply.  I studied physics both to A-level and as part of my degree.  Your answer makes pefect sense but, with respect, it still doesn't explain what I should have said in response to that question from a 12-year-old child!

To Gef:  I don't recall exactly what I said in response to that troublesome question but I think it was roughly along the lines of what you've written!  My problem is that I don't think that it really makes sense to a 12-year-old girl!  I suppose that, in essence, the question asks for a definition of 'silver'.  (i.e. I can define 'red' in terms of a limited range of frequencies and I can define 'white' in terms of a broader range of frequencies but what is it that makes 'silver' different to 'white'?)

[Tim Baxter]

I think it's because.....

The backing of a mirror takes on the flatness of the glass to which it is attached.  It is the silvering which does the reflecting and being effectively highly polished reflects a true image.  Try laying a sheet of aluminium foil under glass to see the effect.

Whereas paper, being matte, is not highly polished and reflects an infinitely distorted image resulting in scattered light.  Paper is only white if you shine white light on it.

[Buenchico]

Another response: Thanks, Tim, for your reply. As I read it, both you and Gef seem to be defining a silver surface as 'smooth and reflecting all visible light' while a white surface is 'rough/matte and reflecting all visible light' but, if these definitions are followed then the surface of a glass of milk would appear to be silver. (Also, according to these definitions, if I spilt some of the paint used for 'silvering' mirrors onto some sandpaper it ought to turn white!).

Sorry, everyone, but I still don't think I've got quite the answer I'm looking for! Further suggestions are most welcome.

[jake-the-peg]

12 year old answer:

Because white objects don't actually reflect like metals do. They absorb the light and re-radiate it. They also re-radiate higher frequency light as lower frequencies - flourescence - which is why white bras show up in discos with ultra-violet light.

Metals such as those backing mirrors actually reflect the same light that hits them immediately.

This isn't the full story by a long shot I think the reflection of light by metals is pretty complex in detail and is due to the "sea" of free electrons which can change to almost any energy state - why gold reflects in a yellow colour is a good follow up question - if you don't mind I'll bow out of that one!  

[Hamish]

In a mirror 80-85% of the light is reflected, in a white material light is scattered. A sheet of glass is slightly reflective (unless treated) say 5% most of the light passes through. If you grind it up you get a white powder, the material is still transparent, but the light is scattered back rather than reflected.

[smorodina]

It's to do with whether the light is directly reflected or scattered. The answer on this page explains it fairly clearly, I think:

http://www.newton.dep.anl.gov/askasci/phy00/phy00399.htm

[ugly_bob]

12-year-old answer - It's magic! ;-)

Actually, better would be that the glass in the mirror is clear and the paper is dyed white.

[Buenchico]

Another response: Thanks to everyone for their time, information and suggestions. I like the input from jake-the-peg best. It goes beyond simplying saying what happens (i.e. the light leaving a mirrored surface is not scattered but the light leaving a white surface is) and explains why. The 'absorption & re-radiation' thing is something which, I now realise, I used to know about but had forgotten.

Just one thing about your explanation for a 12 year old, though, Jake: Are you really suggesting that it would be a good idea for a young, male student teacher to start discussing the flourescence of women's bras with a 12 year old girl? Sounds like a very quick way to end a career to me! LOL!

Thanks again to everyone,

Chris

[Hamish]

Jakes-the-pegs ref to fluroesceance is a effect of whiteners in washing powder and nothing to do with the essance of what whitness is. It is possible to have a white materials wich exhibits no flourescance, similarly it is possible to have a reflective material wich does exhibit flouresceance.

[Hamish]

The problem is with the initial definitition posed by the girl. A white material does not absorb (significantly) and visible wavelengths of light, there is no inherent reflection going on. The material would look transparent, however as it is a powder/poly crystaline/biphasic/id made up of small bits, the light is scattered due to changes in refreactive index and atenuated total reflection processes (ie not all the light gets though, some gets reflected back at you, say about 5%, not enough to make the bulk material reflective.). with a metal the interaction of the incedent light with the electrons in the material result in a reemission of the light and therefore a reflection, usually this is about 80% efficeient.

[Buenchico]

Thanks, Hamish, for your input. I think I'm staring to get confused now! Perhaps we could take it back to the original question. Please, please, please can someone give me the short, snappy, simple answer that I should have to given that kid all those years ago?

Chris

[Hamish]

Things are white because the light is scattered, things are shiny because the light is reflected.

Its a bit like throwing ball at either a railing fence or a wall. Throw at a wall and it will come back at a predictable angle - reflection. Throw at a fence and it may come back but at an unpridictable angle.

This is a very simplistic way of looking at it and the illustration cannot be easily extended as the reason a material reflects/scatters is more complex as you have seen.

[mibn2cweus]

Buenchico, Perhaps these are the facts you require to formulate your answer:

Silver is not a spectral colour; it is not found in the visible colour spectrum. Red, orange, yellow, green, blue, violet, etc., are examples of spectral colours; they can be found it the visible light spectrum.

Silver is a grayscale �colour�; (white), silver, gray, black. Silver reflects all colours, of visible light almost equally, and almost completely; less than white but more than gray or black.

If so, then I am sure you have the skills to organize and communicate these facts coherently, (like a smooth flat reflective surface).

I look eagerly forward to your responce!

[Buenchico]

Hi Mibn2cweus

Thanks for your response. I'm sorry that it's taken so long to reply - I've had only limited time for AB lately.

I'm stiil not totally convinced by your definition of silver as a 'colour' reflecting slightly less light than 'white'. That seems to define 'light grey' to me, rather than silver. It may well be, as suggested by others above, that the nature of the reflecting surface helps to define the difference between 'white' and 'silver' but this doesn't explain how the eye perceives the difference in the surfaces.

Let me put it another way. Imagine that a white light source is shone onto three surfaces. One of them is white, one is grey and the last one is silver. The spectrum of the reflected light is then analysed. I assume that the spectral analysis of the light reflected by the grey surface will have roughly the same pattern as that reflected by the white surface, but with lower intensity. But what about the spectral ananysis of the light reflected by the silver surface? How will this differ from the other two? There must, it seems, be some sort of difference because our own perceptions of colour are obtained by the brain examining the spectral qualities of the light received by our eyes. We see 'silver' differently to 'white' so, irrespective of the qualities of the surfaces under consideration, there must be some sort of difference in the composition of the light itself, in order that our brains can identify the different 'colours'.

Chris

[mibn2cweus]

Chris, You may want to peruse this site. When you do formulate your answer, I am facinated.

[IJKLM]

You know, Buen, many years ago, and on a different board, I had a simlar experience - a friend had asked about primary colours and I spent a while working it all out:

https://everything2.com/user/wrinkly/writeups/Primary+Colors

ANyway, to anwer your question, the white thing is not about specular reflections (as in a mirror). It's about less optically detailed reflections.

And in any case, the 'silver' colour noted in your question is not a colour; it is a kind of sense reaction to the specular reflection.

So th difference is that the specular reflectionin a mirror does not mix up all the wavelengths, but presents all the different frequencies in an ordered fashion, as they presented themselves to the mirrored surface.

When we look at a piece of paper, the different wavelengths are reflected back in a more disordered fashion wit the result that the different wavelengths get mixed up and trigger the white reactio in the observer's eye.

It was only when I realised that the sensatino of colour depends on the receptors in the eye that I worked out how primary colours really work.

There;s nothing special about red or green or blue light. Only about the frequency-sensitivity of the receptors in our retinas.

Does that help?