# Naked Science Forum

## Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: ulivolga on 07/11/2002 06:17:58

Title: color and light
Post by: ulivolga on 07/11/2002 06:17:58
All object are essentially all colors other than the color they appear to be, since the color you see is the one reflected AWAY from the object whilst all other colors are absorbed by the object. My question is... Does an object have color when there is no light? In other words, are my blue socks still blue when I turn out the light?
Title: Re: color and light
Post by: paulat on 07/11/2002 17:10:39
mmm...very philosophical. Bit like "if a tree falls in a forest and there is no one to hear it fall - does it still make a noise !"

PT
Title: Re: color and light
Post by: george on 07/11/2002 17:18:25
I actually think this is quite a good question.

The point is that the 'colour' of an object is the wavelength of light that it does NOT absorb i.e. the wavelength it reflects. Hence if there is no light, the object cannot have a colour.

Also, if the light that you shine on the object does not contain light of the wavelength that the object reflects, then the object appears black - because light of all other wavelengths are absorbed by the object.

This means that when you are buying paint at the DIY shop, for instance, it might look a totally different colour under their lights than it does in your home, because the 'white' light used to illuminate the store might be composed of a different spectrum than the 'white' light at your home.

I think that's right !

George
Title: Re: color and light
Post by: pat on 07/11/2002 17:20:06
George - how can it still be white light if the wavelength is different !??
Title: Re: color and light
Post by: wr8er on 13/11/2002 17:19:39
I think ulivolga creates a few unnecessary confusions. The first statement is actually not right. To correct it you would say that "objects *absorb or transimt* all colors other then the color they appear to be." By making that change, you get a rather obvious statment. But using the word "are" in that sentence throws of all the logic that follows.

The color an object *is* is an intrinsic property: the characteristic absorbtion, reflection and transmission of various wavelengths.  The color that an object *appears to be* is an extrinsic property that depends on the kind (or absence of) illumination and you personal color perception. It can also depend on interference phenomena that create the appearance of color in some feathers and butterflies.

While that version accomodates all of George's examples, I reach the opposite conclusion. Blue socks are blue even when you close the drawer. It is your perception that changes.
Title: Re: color and light
Post by: paulat on 13/11/2002 20:11:38
"Blue socks are blue even when you close the drawer. It is your perception that changes."

Nicely put wr8er...

BUT, does the light go out when you close the door in the fridge ?

Are you SURE that the blue sock is still blue when you shut the drawer ? Perhaps the light falling on the sock makes it change so that it is 'blue'. In the dark maybe the material behaves differently so that it isn't really blue anymore !

PT
Title: Re: color and light
Post by: Exodus on 24/04/2003 15:21:40
concerning fridges. The fridge was invented in 1855 by a Scotsman who moved to Australia... he obviously couldnt stand warm beer. The Scots have done pretty well with their inventors, they have Alexander Fleming too who discovered penicillin (even if it was by accident. ;o)

Thats Economics...
Title: Re: color and light
Post by: chris on 24/04/2003 18:10:40
Someone with some physics know how :

The colour of a surface depends upon which lights are reflected and which are absorbed by the material.

What is the chemical basis behind selectively reflecting light of certain wavelengths in this way ? How exactly does this happen ? Is it a transduction event whereby all light is absorbed by the material which then releases the energy as light of a specific wavelength ?

I'd quite like to get an answer to this if anyone can oblige ?

Chris
Title: Re: color and light
Post by: Quantumcat on 26/04/2003 06:30:05
I think it has something to do with resonance, but I really don't know.

You know how when you are on a swing, if you kick your legs really fast back and forth the swing will go nowhere, and if you swing them really slowly the swing moves slowly. If you swing your legs at exactly the right speed the swing will swing as fast as it is possible.

Perhaps the stuff in the object that makes it coloured has a small enough resonance to be in the range of the vibrations of the light spectrum. Different colours of light hit the stuff and the light of the wavelength equal to the resonance speed of the stuff makes it send the particular colour back out again. (someone pushes the swing back and forth at the right speed to make it go as fast as possible) Light that is not in the stuff's resonance will just be absorbed (Someone shakes the swing quickly) More than two types of "stuff" that have different resonances in the light range produce cyan, yellow or purple. And etc.

That's just a stupid theory I made up it's probably not right !!!
Title: Re: color and light
Post by: tweener on 15/08/2004 17:14:58
I'm gong to bring this thread back from the dead as the question has not been addressed properly.  Quantum has the right idea but there is a more exact explanation.

Here are my thoughts, but there are others on the forum with better knowledge than mine.

The molecules that make up a material have a certain configuration of electrons on the outside.  When a photon of a certain wavelength (corresponds to color) interacts with the electrons on the molecule, it may be absorbed or reflected or not affected at all.  If it is absorbed, while photons of other wavelenghts are not, then that color is subtracted from the light and a color cast is given to the reflected or transmitted light beam.  If the light illuminating the sock is "white", then absorbing most of the red colors will result in a blue sock.  If a photon is absorbed, that means that an electron has been boosted to a higher energy level.  This energy is released later as another photon (possibly of a different wavelength) or possibly as a phonon (quantum vibration) in a crystal.

But not all light sources are white.  We usually consider the sun to be "white" though it has a yellow cast if you look at the power spectrum (more yellow photons than blue or red).  Tungsten light bulbs give off a  very reddish-yellow color light with a fairly wide spectrum.  Flourescent lights are a sickly green with lots of components, but a few strong lines of color.  Sodium vapor street lights are a strong yellow color that is very tightly grouped in a narrow spectrum.
As George mentioned above, if you have an object that absorbs the primary color of the light source, but the source doesn't have other colors, then the object looks black.  A good example of this is something red viewed under mercury or sodium streetlights.  It's not red anymore - it's black.

----
John - The Eternal Pessimist.
Title: Re: color and light
Post by: OmnipotentOne on 17/08/2004 01:03:34
hmmm I remember the 5 phenomenas going on are reflection, refraction, absorbtion, scattering, and polarization.  EM fields and dielectrics, i never really understood it all properly.... but DAMN that really makes you think, maybe my socks really do match in the end!

To see the world through a grain of sand.
Title: Re: color and light
Post by: McQueen on 27/08/2004 00:51:51
Isn’t it peculiar that in all these models of colour and human perception of colour , no mention is ever made of the intensity of the colour in question. Surely , the intensity of light should play a pivotal role in what colour an object is ? We all know that such a concept as intensity does exist , yet there is no commonly accepted  definition of the intensity of light.  Very often the only conclusion that can be reached is that the intensity of a given light source depends upon its energy ( frequency ) thus as far as light is concerned the eigen value and the frequency of the light in question  are taken to be the same. This cannot be right. If the intensity of colour is taken into account in our perception of colour , then how does it work. In our computer monitors , intensity plays a vital role in the number of colours which can be displayed. In prints and photographs on the other hand it is the distribution of dots of  magneta , indigo and cyan which reproduce colour. The question is how does the human eye perceive colour is it through the print model or through the computer model. The second concept which is confusing is the notion that light of certain wave-lengths  is “absorbed” while light of other wave-lengths is reflected. Where does the extra energy absorbed by the electron “go”. The electron always tries to retain its ground state energy , if light is absorbed what happens to it ?
Title: Re: color and light
Post by: tweener on 27/08/2004 03:08:06
The perceived intensity of the light is a function of how many photons are hitting the retina. The photons excite the sensory neurons, with more photons causing more rapid firing of more neurons.  The eye has three types of color sensors (cones) that are sensitive to red, green, and blue light.  The different intensity of these three colors at the same time is responsible for our perceiving all different colors and increasing the number of photons for all increases the intensity of the color.  The intensity of a light source is determined by the number of photons it emits.  Changing the wavelength will change the color of the source.

There are also a different type of sensor (rods) that is not responsive to color but is more sensitive to light, thus making it very difficult to distinguish colors in very dim light, even though you can see the object.

The difference between monitors and printers is that monitors emit light of three colors - red, green, and blue (as you have said).  Printers use cyan, magenta, and yellow because they are operating on the absorption rather than emission of light.  Also, since inks are not perfect, an equal mixture of CMY will not usually result in black (as it theoretically should) and so the printer also uses black ink to make images have proper blacks.  Some high-end printers now have seven colors, the standard CMY&K and light versions of CM&K (I guess they don't need light yellow).

The energy in a photon that is absorbed by a substance can be re-emitted later as another photon (probably of a different wavelength) or it can simply be converted to vibrational energy in the material, commonly known as heat.  That's why black things in sunshine get hot faster than white things - they are absorbing photons and converting them to heat.  Some of it is being radiated away as infra-red, some of it is being convected away by air (or your hand when you touch it).

Scattering is really reflection but in random directions.  We usually think of reflection in the sense of a "coherent" reflection that obeys Snell's law at a macroscopic level, like a mirror.  Scattering is the same process but on a small scale the photons are being reflected in random diretions.  A white sheet of paper is a good example - you might get 95% of the light reflected, but you can never use it as a mirror because the light is going all directions.

I think I'm just rambling here.  I hope this helps, but if not, I'll give it another swing.  Or not - your choice.

----
John - The Eternal Pessimist.
Title: Re: color and light
Post by: McQueen on 27/08/2004 10:57:37
Printers use cyan, magenta, and yellow because they are operating on the absorption rather than emission of light.
Thanks Tweener , that  reply was quite helpful. There are one or two points however, which I would like to clarify. This question of absorption  of colours in the CMYK system has been troubling me for some time.  When one wants to reproduce a colour photograph the procedure is as follows. The photograph is first photographed through red and blue filters. This forms the Magenta plate , next the procedure is repeated with red and green filters , this gives the cyan plate lastly the procedure is repeated with green and blue filters to give the yellow plate.  The interesting thing to note is that on the magenta plate , the magenta will appear in places which are predominantly red , the same with cyan which appears in green and blue areas. I would have thought that it should be the other way around (this is getting complicated ) i.e cyan appearing in red areas so that it absorbs all the blue and green  leaving the red and so on. In colour negatives  it actually does work that way , of the three CMY layers , when photographing a blue object only the CM layers will be activated , blocking out red and green light and allowing the blue to pass through the yellow (complement of blue ) layer , showing the object up as blue.
So in effect when looking at a colour print we are dealing almost totally with reflected light , green being a mixture of cyan and yellow dots and so on.  Where does the absorption come in ?
Title: Re: color and light
Post by: gsmollin on 27/08/2004 15:33:40
I'm not following the question, it did indeed get complicated. Break it up into quick bites.

Reproducing a color print, photgraphically, usually requires another photograph be taken with color negative film. The color negative process is more complicated than just the "negative" of the primary colors, because it is an intermediate step to printing. There is compression of contrast and an overall orange mask which complements the response of the emulsion on the printing paper. When you are done you have a print that you see by reflected light, so subtractive color mixing applies.

The CMYK color printing-press process, not photographic, is even more complicated because the colors we see are formed by half-toning patterns of primary colors on the page.

We need to keep these two types of image systems separated in any question, because the answer is rather different for each.
Title: Re: color and light
Post by: McQueen on 28/08/2004 15:42:13
The CMYK color printing-press process, not photographic, is even more complicated because the colors we see are formed by half-toning patterns of primary colors on the page.
Right let's stay with the CMYK model . Where does the absorption come in? Green we see as a mixture of cyan and yellow dots , similarly red would be magenta and yellow dots and son . But surely thsi has everything to do with reflection and nothing at all to do with absorption?

Title: Re: color and light
Post by: Quantumcat on 30/08/2004 00:33:14
It has everything to do with both absorbtion AND reflection, since you have to do both to see the intended colour. To get green you'd have to subtract all but green, cyan and magneta, so you'd have blue and yellow dots, blue subtracts magneta (blue made of cyan and green) and yellow subtracts cyan (yellow made with green and magneta). So you're left with green. That's why it's called "subtractive colour mixing" it's what you do when you paint, you mix the colours that will absorb all the colours except the ones you want to portray. When you take a photo, you put in the three filters to get the colour you want to go through, but that makes it so you get the colour that absorbs the colour you want and reflects the other colours you don't want. So when you make a positive from the negative you switch the colours on the paper so you absorb what was reflected and reflect what was absorbed. I have no idea if that's what you wanted to know or if that was already obvious beforehand and you wanted a deeper answer, but I did the best I could. lol I don't even know if I'm right (possibly not)

As to the colour question I think that it depends how you define "colour". I think that an object still has colour even when there's no light because what colour an object reflects and absorbs is intrinsic to that object, to the chemical makeup in its dyes, and that we simply percieve this intrinsic property with the medium of light. Colour is an object's own property and not one of the light that carries it. An object can't suddenly gain a property just because you happen to turn on the light. It's like the old saying "don't kill the messenger" it's not from the messenger that the message comes, just because it's through that that we percieve it.

Am I dead? Am I alive? I'm both!
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.stupid-boy.com%2Fsmilies%2Fkao%2Fotn%2Fcat.gif&hash=e4b91a72c020cc1c5d28487fff5428f1)
Title: Re: color and light
Post by: McQueen on 30/08/2004 10:26:52
Quantumcat

It has everything to do with both absorbtion AND reflection, since you have to do both to see the intended colour. To get green you'd have to subtract all but green, cyan and magneta, so you'd have blue and yellow dots, blue subtracts magneta (blue made of cyan and green) and yellow subtracts cyan (yellow made with green and magneta). So you're left with green. That's why it's called "subtractive colour mixing" it's what you do when you paint, you mix the colours that will absorb all the colours except the ones you want to portray.

I understand and basically agree with what you have to say. Where I get confused is that when you look at a color print through a magnifying glass , what you see ( to take your first example ) are not green dots , but cyan and yellow dots. How does this work because clearly there is no absorbing going on since , through a magnifying glass we can see the actual colours of the cyan and yellow dots, yet when the magnifying glass is removed the dots immediately resolve themselves into green.  How does this happen , one possible solution is that just as the whole spectrum of colours are contained in sun light and through a process of interference manifest as white light , is it possible that light reflected from cyan and yellow dots similarly experiences interference and reaches the eye as green light. Its pretty confusing.  Because if we extend your theory what it should mean is the cyan dot should appear green even through the magnifying glass , otherwise it would mean that the cyan dots only absorb light of a certain colour when we are not looking ! Sorry for being so obtuse ,  what can one do but keep chugging ?
Title: Re: color and light
Post by: gsmollin on 31/08/2004 14:15:22
I think I'm understanding McQueen's question now, and it is indeed a good one. Let me re-state it, as I understand it.

We can form colors on a screen or page by mixing primary colors. The tri-tone primary colors are red, green, and blue for additive color mixing. So on a TV screen, we can look with a magnifying glass and see red green and blue dots or stripes on the screen being used to make up all the colors. There is quite a lot of information available on the web about this process, if we search on "NTSC color standard".

The tri-tone primary colors are cyan, magenta, and yellow for subtractive color mixing. So we can mix these colors of paint or ink to get "con-tone" images, such as on a photographic print, or an artist's painting. Another application would be "spot" color from a printing press.

Now we come to the question: Half-tone images are constructed of dots, much like the screen of a TV set or computer monitor. Why are the dots colored cyan, magenta, and yellow instead of red, green, and blue? Think before you answer! Each dot is separate from its neighbor, and the reflected light from each and every dot travels an independent path. So there is no color pigment or ink mixing, such as in a painted image or a photographic print. In addition, the dot sizes are very large, say .007 inches (.2 mm), so interference cannot be a significant issue. Why are the primary colors different, CMY, instead of RGB?
Title: Re: color and light
Post by: roberth on 01/09/2004 00:11:26
Actually, the primary colours are red, yellow and blue. Green is a combination of blue and yellow. I think cyan and magenta are just names used by computer geeks to try and confuse the rest of the population.
Title: Re: color and light
Post by: gsmollin on 01/09/2004 19:34:00
No, computer geeks get blamed for a lot that's not their doing. The additive primary colors are red, green and blue. Look on your computer monitor with a magnifying glass if you don't believe me.

The subtractive color primaries are also cyan, magenta, and yellow. Red looks similar to magenta, and blue looks similar to cyan, but they are not the same.

There are a whole set of relationships between the additive primary colors, and the subtractive primary colors, but you can research that for yourself, if you are interested.
Title: Re: color and light
Post by: Ultima on 01/09/2004 19:48:35
roberth primary colours for pigment and light are different.

Pigment: yellow, cyan, magenta. Cyan being light blue, and magenta being a purple. Look at a colour newspaper under a magnifying glass to see.

Light: Red, Green, Blue. Check out the pixels of a TV or monitor with a magnifying glass to see this one.

Found a good google'r to illustrate:

http://homepage.mac.com/dtrapp/physics.f/ColorVision.html

wOw the world spins?
Title: Re: color and light
Post by: roberth on 02/09/2004 00:42:25
Geez, I know I'm getting old, but when I was younger, the primary colours were red, blue and yellow. I guess computers have really changed the way we look at things. When I colour in the sky (blue or is that azure or cyan) with my colouring in pencils and put a yellow sun on there, it still comes out green, so not everything has changed.
Thanks for the insight, guys.
Title: Re: color and light
Post by: gsmollin on 07/09/2004 18:28:32
quote:
Originally posted by gsmollin

I think I'm understanding McQueen's question now, and it is indeed a good one. Let me re-state it, as I understand it.

We can form colors on a screen or page by mixing primary colors. The tri-tone primary colors are red, green, and blue for additive color mixing. So on a TV screen, we can look with a magnifying glass and see red green and blue dots or stripes on the screen being used to make up all the colors. There is quite a lot of information available on the web about this process, if we search on "NTSC color standard".

The tri-tone primary colors are cyan, magenta, and yellow for subtractive color mixing. So we can mix these colors of paint or ink to get "con-tone" images, such as on a photographic print, or an artist's painting. Another application would be "spot" color from a printing press.

Now we come to the question: Half-tone images are constructed of dots, much like the screen of a TV set or computer monitor. Why are the dots colored cyan, magenta, and yellow instead of red, green, and blue? Think before you answer! Each dot is separate from its neighbor, and the reflected light from each and every dot travels an independent path. So there is no color pigment or ink mixing, such as in a painted image or a photographic print. In addition, the dot sizes are very large, say .007 inches (.2 mm), so interference cannot be a significant issue. Why are the primary colors different, CMY, instead of RGB?

Title: Re: color and light
Post by: McQueen on 08/09/2004 01:14:30
Gsmollin

I almost missed your post. Thanks for re-posting. I think that the mistake we are making is in trying to put labels like “subtractive” and “additive “ to the RGB and CMY colour systems. The reason  I say this is that for the CMY system to be a truly  subtractive system , colours would have to be  put down in layers , so that certain colours are absorbed by certain layers of colour  allowing  other colours to be reflected. In practice this is not what occurs. Paints are colloidal substances , that is they consist of a suspension of pigment particles in a medium. When a painter wants to obtain a certain colour , he doesn’t put first one layer of colour on and then another , what he does is select certain amounts of each colour and mixes them on his palate , what finally reaches the canvas is a mixture of the two colours or spots of pigment of the two colours  which gives rise to  a third colour. Similarly , and this is easy to see with a magnifying glass , Colour photographic prints , are also made up of dots in this case Red , Green and Blue dots. So this discussion has shown that in every case exactly the same system is at work. We have dots of either the primary or the secondary  colours , which get mixed together to give rise to other colours .  This process must necessarily be due to the reflection and interference from the different dots before they reach the retina and not due to any “subtractive”  process ,  although in a sense the “additive “ process is still viable because interference might be considered as being a form of addition of light of different colour.
As to the question of why two systems of colour exist , RGB and CMY .  I think you have pointed out the answer yourself when you had talked about the relative coarseness of the grains found in half-tone images. Since RGB are clearly defined colours , the reproduction using the coarse half tone size of grains would not result in good reproduction. By using secondary colours which are  milder almost pastel shades , good reproduction is achieved in spite of the relatively large grain size.
Title: Re: color and light
Post by: gsmollin on 09/09/2004 17:51:34
Actually, I was posting an answer to my question, but it never happened. Here is what I meant to say:

R+G+B=white, so these are the additive primary colors. This is true if they are truly mixed, say using prisms, mirrors, or lenses. It is also physiologically true if they are mixed by showing dots too small to be resolved by the eye at a distance. The dot pitch on your computer monitor, for instance is about 0.26 mm. With a magnifying glass you can see the red, green, and blue dots. The use of additive mixing in CRT screens is a natural for the way a CRT works. When the electron beam is off, the tube looks black. The surface of the tube has a black coating on it to make it look black. The RGB guns light up the RGB phosphors, and we see colors.

The opposite is true of the photo, painting, or half-tone-printed page. The paper is white, so to see colors, we must subtract the colors we don't want to see. This is obvious to anyone who has colored or painted. The problem I was having was with half-tone printing. It is a mosaic of CMY dots and other shapes. (Many times special colors are used, however, so you must be sure you are looking at a CMY-half-tone-printed image) But these dots reflect light individually, so their contributions are added- giving us additive color mixing. That was my point of confusion. The apparent answer to this question is that the subtractive primary dots are combined by our visual process, and appear to us as if they had been mixed together like paints.

I imagine the visual process works something like this:
The light from the C, M, and Y dots all falls on 1 cone cell, since the dots are too close together to be resolved individually. The cone cell responds as if the C, M, and Y inks had been mixed together, since it sees them as one. The result is subtractive color mixing. The same visual process is happening for additive color mixing.

OBTW, if also found out that "k" is used for black, since "b" was already used for blue. Duh.
Title: Re: color and light
Post by: McQueen on 10/09/2004 07:28:38
An interesting corollary to the theory which I have put forward  is that all light is due to absorption and re-emission of photons of  a particular colour. The proof of this is very simple , just look in a mirror !!  A mirror as we all know is made up of a glass  sheet generally  of from 3 – 5 mm thickness at the back of which is a silvered layer. Now , it is common knowledge that light travels through substances like glass by being absorbed and re-emitted by electrons in the glass , therefore in order to see the colours of an image reflected in a mirror , the photons of those particular colours must first journey through the glass being in turn absorbed and then emitted by electrons in the atoms making up the glass , till they reach the silvered surface where they are absorbed and re-emitted one final time before making the reverse journey back through the glass from atom to atom and from there to the retina. The question of why the silvered layer reflects light back in the direction from which it has come is due to the properties of metals but it seems that this property must be shared at least partially by all substances , although it is obvious that with metals all light is emitted and absorbed while other substances selectively absorb and emit light.  Thus it is plain to see that all colours are due to a process of absorption and emission of photons of the requisite wave length.  Now supposing we have an object which is pure blue under white light what is happening ? We have to assume that the object is made up of atoms whose electrons are particularly susceptible to photons with the energy and wave length  of blue light (around 550 nm ) and practically ignore all other wave-lengths and energies. We know that something very much like this must happen from our study of spectroscopy , where certain substances only absorb and emit light of certain colours , for instance sodium will only emit and absorb yellow light , ignoring all other wave lengths and energies. No-one would suggest that sodium is absorbing all the photons of other colours and reflecting only the yellow light , in fact it is simply ignoring the other photons and selectively absorbing and emitting yellow photons ! Returning to the case in question of the blue object , we must assume that when white light shines on this surface , the electrons of the atoms making up this object absorb and re-emit only the blue light so that the  object looks blue. This is an intrinsic property of the  object , thus it retains this property even when there is no light. However if light of a different frequency , (as for instance ultraviolet light ) falls on the surface , it activates different electrons with the result that photons of a different wave-length are absorbed and emitted , while the blue photons which were predominant under white light are suppressed , and the object changes colour. The most interesting part of this discussion so far as I am concerned lies in coming to an understanding of just how fast and how continuously electrons emit and absorb photons under the influence of radiation even by  white light . The emission and absorption of photons by electrons under the influence of radiation  is seen to be a practically   continuous and unremitting process. It is difficult to imagine while looking into a mirror or through a window ( this for those who  still have reservations  about my ideas )  the number and speed at which these emissions and absorptions of photons  must  take place in order to form the continuous  images which we perceive.