What decides the colour of light?

[Rishi Gangadhar]

Consider a beam of light passing through a slab of some refractive index.
We know that the speed and wavelength of the light changes, but its frequency remains the same.
Since the wavelength of the light changes, does its colour change, or does it remain the same as its frequency remains the same.

 

[DrClaude]

Color only has meaning when the light reaches the eye. All that counts is the frequency.

 

[sophiecentaur]

Although people tend to talk in terms of the wavelength of light, that quantity keeps changing as it passes through different media (in particular, it is different whilst it is actually in the eye). Frequency is not changed (as Dr Claude pointed out). But pretty well all the light we see consists of a mixture of frequencies. We seldom come across monochromatic light in nature. Apart from lasers and some electric discharge lamps, the light is far from pure.
It is vital to distinguish between colour and wavelength at all times.

 

[UncertaintyAjay]

Correct me if I'm wrong, but colour is, essentially, the minds way of detecting frequencies. Instead of X Hz or Y Hz, our brain recognizes different frequencies as different colors.

 

[sophiecentaur]

Correct me if I'm wrong, but colour is, essentially, the minds way of detecting frequencies. Instead of X Hz or Y Hz, our brain recognizes different frequencies as different colors.

A partial correction. There is a combination of frequencies involved with most colours.( the ones that are not "spectral" colours). Think in terms of musical chords rather than just tones.

 

[phinds]

...
We know that the speed and wavelength of the light changes, but its frequency remains the same ...

... the wavelength of light, that quantity keeps changing as it passes through different media (in particular, it is different whilst it is actually in the eye). Frequency is not changed ...

HUH? Frequency and wavelength are inverse properties. How can one change and not the other?

 

[phinds]

Correct me if I'm wrong, but colour is, essentially, the minds way of detecting frequencies. Instead of X Hz or Y Hz, our brain recognizes different frequencies as different colors.

I think it's probably more correct to say that color is how the brain categorizes the electrical signal sent to it by the optical nerve rather than that the brain "detects frequencies" directly.

 

[UncertaintyAjay]

Think in terms of musical chords rather than just tones.

Love the analogy . Thanks.

Frequency and wavelength are inverse properties. How can one change and not the other?

Because velocity decreases. Sophiecentaur was referring to light traveling through different media.

 

[nasu]

HUH? Frequency and wavelength are inverse properties. How can one change and not the other?

The proportionality constant (speed of light) changes so that the frequency stays the same.
The wavelength changes due to the changes in speed of light in various media.

 

[phinds]

The proportionality constant (speed of light) changes so that the frequency stays the same.
The wavelength changes due to the changes in speed of light in various media.

Got it. Thanks. I'm slow today. Well, OK, I"m slow every day, but ...

 

[sophiecentaur]

I think it's probably more correct to say that color is how the brain categorizes the electrical signal sent to it by the optical nerve rather than that the brain "detects frequencies" directly.

I always say that human colour perception is a really poor spectrometer. It is sooo easy to fool. And it doesn't matter at all.

 

[phinds]

I always say that human colour perception is a really poor spectrometer. It is sooo easy to fool. And it doesn't matter at all.

I disagree that it doesn't matter. There are situations where subtleties in color do matter.

 

[sophiecentaur]

I disagree that it doesn't matter. There are situations where subtleties in color do matter.

Of course - but the eye is still a lousy spectrometer. That is what doesn't matter because it is never called on to do that job. Colour is not wavelength, is it?
The eye is very good at resolving small differences in perceived colour (when it matters) but that doesn't involve measuring wavelength but combinations of the outputs of just three groups of sensors.

 

[artyb]

Of course - but the eye is still a lousy spectrometer. That is what doesn't matter because it is never called on to do that job. Colour is not wavelength, is it?
The eye is very good at resolving small differences in perceived colour (when it matters) but that doesn't involve measuring wavelength but combinations of the outputs of just three groups of sensors.

This seems similar to Edwin Land's Retinex theory:

"The Retinex Theory of Color Vision

Λ retina-and-cortex system (retinex) may treat a color as a code for a three-part report from the retina, independent of the flux of radiant energy but correlated with the reflectance of objects "

The Retinex Theory of Color Vision SCIENTIFIC - CiteSeer

 

[phinds]

Of course - but the eye is still a lousy spectrometer. That is what doesn't matter because it is never called on to do that job. Colour is not wavelength, is it? The eye is very good at resolving small differences in perceived colour (when it matters) but that doesn't involve measuring wavelength but combinations of the outputs of just three groups of sensors.

Ah. That I agree with. There are people who can reliably discern very subtle differences in color (which is what I mean that mattered) but if they were to look at one of those colors one day and one close to it the next day I doubt they could tell the difference.

 

[sophiecentaur]

This seems similar to Edwin Land's Retinex theory:

"The Retinex Theory of Color Vision

Λ retina-and-cortex system (retinex) may treat a color as a code for a three-part report from the retina, independent of the flux of radiant energy but correlated with the reflectance of objects "

The Retinex Theory of Color Vision SCIENTIFIC - CiteSeer

I remember reading that paper a long time ago when I was involved in colour TV. The well known tristimulus theory of colour vision seems to overlap the Retinex Theory. Reflectance is not the only thing that counts for a lot of our visual input these days (TV displays and projected film) and it is altogether a very complicated business. The eye manages to process out things like the illumination in assessing the colour of an object. That is truly amazing and the description of the process - 'integrating to grey' is a bit of an oversimplification. It's about all that your automatic digital camera colour correction can manage. The eye seems to extract information at a far deeper level, based on context and memory. Brilliant and the Land paper makes a good effort at describing what goes on.

 

[DaveC426913]

... combinations of the outputs of just three groups of sensors.

Or four, in some rare cases...

 

[sophiecentaur]

Or four, in some rare cases...

Tetrachromacy is one extreme of colour vision, I guess. Colour vision is a very personal thing and those tristimulus response curves (look em up folks) are the result of a lot of statistics, conducted on a lot of subjective results with a lot of people (probably a limited racial spread, though), I believe. There is much more spread in the responses between different people than your average conversation acknowledges. The system was aimed mostly at getting a good enough display and printing method to satisfy enough people with the 'accuracy' of copied colours. The proof of the pudding seems to suggest that RGB and CMY depiction of colours is 'near enough'.
One of these days, perhaps, someone will come up with a TV system that uses more than three analyses and more than three basic phosphors. That could be really impressive and we would then start to realize the limitations of what we have at the moment. Colour printing just can't get away with three colours when the colours really count.
There are parallels with Stereoscopic displays, which are impressive but very limited, in fact and multi channel surround sound systems which do better than bog standard stereo sound.
Our brains are definitely on the side of the manufacturers, though. They desperately try to get sense out of these artificially presented sensations, despite the imperfections in the reproduction system.

 

[UncertaintyAjay]

I always say that human colour perception is a really poor spectrometer. It is sooo easy to fool. And it doesn't matter at all.

Its still pretty damn impressive for all that. And as a spectrometer it serves us very well.

 

[sophiecentaur]

Its still pretty damn impressive for all that. And as a spectrometer it serves us very well.

Of course, our colour vision serves us very well - in terms of our survival, and we should not expect anything more than that. (Evolution / Nature never does more than necessary). As a spectrometer, the eye is actually totally inadequate. It cannot even tell the difference between spectral Yellow and a combination of two monochromatic Red and Green lights. If I bought a spectrum analyser that could be fooled as easily as that then I would send it back to the shop.
I don't understand why people get all defensive about their bodily system when someone points out its inadequacies. Our vision is what it is. It has no evolutionary advantage in being a spectrometer - so it never developed to be one.
"It doesn't mean you're a bad person."

 

[wud-wurks]

The cones in the retina of the eye are stimulated this way, 64% react to red light of frequency centred about 650 nanometers about 33 % react to green light centred on 540/550 nanometers and just 2 % react to blue light centred on 450nanometers. SO if you look at a banana for example, all but the frequencies between 570 and 580 are absorbed and the 570/580nanometer light is reflected to your eyes. The cones respond as indicated and the signals in terms of amplitudes from the cones are transmitted down the optic nerve to the brain. The brain interprets these amplitudes/ decodes them if you will, and responds with the result that you are looking at something with a colour we call yellow.

 

[sophiecentaur]

A very nearly monochromatic bannana skin? Hardly likely. It's surface colour, under white illuminant will probably sit around half way between white and spectral yellow on the CIE chart. The pigment will probably be a mix of several natural dyes - at least it could well be. It sure ain't spectral.
If I'm being picky, it's to raise a bit more awareness about the nature of colour.

 

[nasu]

Some people actually study reflection spectra of bananas. :)
http://ucce.ucdavis.edu/files/datastore/234-953.pdf
The graph in figure 3 tells it all.
For a well ripened banana, the reflection coefficient (for visible light) is maximum in the range 550-680 nm. And is at least 20% for the rest of the visible range.
One of the pigments is chlorophyll but obviously, not the only one.

 

[sophiecentaur]

Some people actually study reflection spectra of bananas. :)

And probably the curvature, too. (I just watched the Hugh Fearnley-Whittingstall programme on Cosmetic vegetables; we are so very fussy about curvature in veg)

That spectrograph is interesting as it shows a peak that is 'identifiable' as 'a yellow' (as expected). We would all agree that bannana colour could be described as a bright or strong yellow. However, looking at the total area of the rest of the curve, in the visible range, it is nearly the same as the area of the portion that you could describe as yellows. So, it's fairly desaturated and far from spectral.
I was looking in my massive photo library for a convincing picture of a bannana so that I could look at the RGB components of its yellow colour. I haven't found a picture yet but I may take the trouble to photograph one later today. If it's anything like the other bright coloured objects I have on file, there will be very significant B contributions, along with the G and R (which produce the recognisable yellow). Looking at a brightly clothed audience at an outdoor sport event on a sunny day, it is hard to find objects that are actually 'saturated' colours. Of course, your TV display will never give you spectral colours because they lie outside the gamut, encompassed by the phosphors.

 

[ToddSformo]

Are you saying that it's more proper to note that we are seeing
red at 400–484 THz rather than seeing at 620–750 nm. I know these are equivalent, but in terms of perceived color we are responding to frequency?

 

[phinds]

Are you saying that it's more proper to note that we are seeing
red at 400–484 THz rather than seeing at 620–750 nm. I know these are equivalent, but in terms of perceived color we are responding to frequency?

Your optical nerves respond to how often they are hit by the peak/trough of a wave. Why would they care how far apart the peaks are as they travel towards your eye?

 

[nasu]

As long as is understood that the wavelengths are in air there will be no confusion. The practice of using wavelengths is already well established and the numbers (wavelengths) are easier to remember, I think.

Of course, these wavelengths do not apply to the light actually reaching the retina. It is not in air, I suppose. But it does not matter.

 

[sophiecentaur]

Are you saying that it's more proper to note that we are seeing
red at 400–484 THz rather than seeing at 620–750 nm. I know these are equivalent, but in terms of perceived color we are responding to frequency?

When light was first studied, they couldn't measure the frequency (or even be sure what c was, exactly). However, any Tom, Dick or Isaak could measure the wavelength of the light he was using to a high degree of accuracy, starting from scratch. So wavelength was, and still is, the common currency. But chemicals, sensors and other systems of microscopic charges, work on Energy, which is best quantified in terms of frequency. So both your alternatives are fine. It is the Photon energy that your receptors work on - but you could say that the 'optics' (lens etc) are basically wavelength orientated.

 

[Don Jusko]

Got it. Thanks. I'm slow today. Well, OK, I"m slow every day, but ...

Me too, I've got to get this physics down. This topic was up my ally, it's not a street yet, so this is what I started with.
Science Advisor said a spectrometer could tell the difference between making yellow from two lights, red and green, and the spectrum pigment color.
That being said, I would like to know the difference between the printed yellow photographed and actual pigment yellow, and, the difference between the two yellows examined with a spectrograph. If there is any difference at all I think it is the fault of the electrical receptors in the camera and spectrometer, the cadmium element making spectra yellow can't be wrong but the photo mechanical electrical results can be. I wish I had a spectrometer to compare all three yellow's.
http://www.realcolorwheel.com/final.htm

 

[sophiecentaur]

I can't read your attachments, I'm afraid.
There are a million and one ways of synthesing any particular colour (i.e. producing a match). Your "actual pigment" can be a mixture of natural substances but even a single substance will not reflect only spectral yellow. If it only reflected a very narrow band of colour then it would, unfortunately, look very dark so it can be a very difficult job. This is one reason that they use 'spot colours' in colour printing, because they can end up brighter ( and a better match - say to the coca cola red) than when made with the basic palette of inks)
Cine film was a nightmare to get right and for the reels of different stock to be made near enough so that the audience couldn't spot the reel change. But it was found that people are not as fussy about colourimetry in a dark cinema than they are in their own homes, watching TV, with familiar colours all round them in their living rooms.