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### Re^5: I'm not a PhD but...

by BrowserUk (Patriarch)
 on Feb 04, 2009 at 17:10 UTC ( #741358=note: print w/replies, xml ) Need Help??

in reply to Re^4: I'm not a PhD but...
in thread How many colors does a rainbow have?

As this diagram shows, the responses of the cones overlap,

I'm very aware of that as you'll see if you follow the 3rd link in this post.

This assumes that only one cone will respond to a specific frequency.

Okay. The "colors" through which each point would scintillate wouldn't be red, blue and green. But, assuming the light eminating from any given point source consisted of an unattentuated, pure frequency, then if what we percieve were in direct proportion to that pure frequency, then the points would scintillate as the focused light hit the three diferent types of cones. And the three colours through which they would scintillate would be the same for every source of that given frequency.

But that doesn't happen. In part because of the persistance of human vision which aggregates and averages the spectral responces of many individual cones of all three types over time. To quote from another link in that other post:

Any color on the CIE chromaticity diagram can be considered to be a mixture of the three CIE primaries, X,Y,Z. That mixture may be specified by three numbers X,Y,Z called tristimulus values.

The CIE primaries are not real colors, but convenient mathematical constructs.

Nevertheless, the tristimulus values X,Y,Z uniquely represent a perceivable hue, and different combinations of light wavelengths which gives the same set of tristimulus values will be indistinquishable in chromaticity to the human eye.

So, whilst there may be no adjacent bands of red and blue in the source spectrum, our perception of all hues (including "pure red" and "pure blue"), will have influences from the conal responses to incedental and coincident frequencies eminating from other sources over both space and time. Ie. We percieve pinks in a rainbow even though there are none there at the source.

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

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Re^6: I'm not a PhD but...
by tilly (Archbishop) on Feb 04, 2009 at 18:51 UTC
I really do not understand why you would think that the colors would scintillate. Perhaps you are looking at the detailed mechanics that cones fire, recover, and fire again? In which case yes, persistence of vision is responsible for smoothing that out. And therefore reconstructs in our brain the impression of a constant mix of colours, which just happens to closely match the external reality of light that will create a continuous mix of reaction by our cones and rods. (Persistence of vision is famously responsible for allowing a sequence of still pictures to look like continuous motion - see your local television or movie for an example.)

Furthermore it certainly is the case to a reasonable approximation that a given frequency of light is the same colour. That's not exactly true because our perception of colour is mediated by whatever else we are seeing at the same time. This is necessary so that an object appears to remain the same colour as we carry it from a reddish indoor light to far more blueish outdoor light. As we do so the light we see changes substantially, but it still looks the same to us. But let's ignore that complication.

The important point is that I really don't understand why you think we should anywhere perceive pink in the rainbow. Particularly since I don't see it when I look at the rainbow.

Pink results from firing red and blue receptors without firing green. But given the response curves of our receptors, there is no frequency that does that because green responds strongest to colours between what red and blue respond to. This is is easily tested - just look at a rainbow. And when I do so, nowhere do I see pink. Which is exactly what theory tells me should happen.

I really do not understand why you would think that the colors would scintillate.

Image a single point source of pure red light coming to our eyes from an otherwise completely black vista. (Think:a laser sight aimed at us in a dark room.) As we move our eyes, (or they twitch with our pulse), the photons from that (for arguments sake impossibly fine), point source would alternately fall on "red" cones, "green" cones and "blue" cones.

As we move our eyeballs, the image in our brains doesn't move. The brain maps the tristimulus values originating from different (and widely spaced) cones, back to the same point within our mental image. Without persistence of vision, our perception of the colour of that single point in our mental image, would change as the photons originating from it stimulated different cones with their differing spectral responses. But that doesn't happen.

With persistence of vision, the colour we perceive coming from that point source is an averaged combination of the responses generated by the photons falling on lots of different (and different 'coloured') cones over time as our eyes move. In this artificially simple case, the colour we perceive in our brain at the point source, is an amalgam of the tristimulus responses from all the cones that those pure "red" photons hit as our eyes move, and we attach the label "red" to that amalgam.

In the real world there are no pure frequency point sources. The photons reaching our eyes from any given point (or rather, from the same directional vector), will be a mixture of many frequencies. In the case of a rainbow, given the distance between our eye and the raindrops that are responsible for the rainbow, we will be unable to resolve anything less than (say) a square foot, or a square metre, or maybe larger in the XY planes. And indeed, we mustn't forget that the raindrops that form the appropriate angle between our eyes and the Sun will not all be exactly in the same plane in the Z axis. And the raindrops themselves are falling at some rate.

So the light reaching our eye and being resolved to a single point in our mental image within any given period of persistence, will have come from some or all of the raindrops that passed through a cube (or sphere) of air 10s or 100s of miles away. And there will be some light being reflected from the outsides of raindrops both further away and closer to us that will arrive at our eyes coincident with the refracted light. And some of the spectral components refracted will have been attenuated by the air and water molecules between the refracting raindrop and our eye.

The colour we perceive will be the result of the amalgamation of all those stimulations within the period of persistence.

The important point is that I really don't understand why you think we should anywhere perceive pink in the rainbow. Particularly since I don't see it when I look at the rainbow.

Well, the first question to ask, is when was the last time you actually looked at a real rainbow with your own eyes?

As opposed to a instantaneous frozen image of one interpolated through a CCD device with some number of bits of A/D conversion, thence further manipulated (reduced) into a jpg, png or (if you are lucky) tiff format file, and probably resized with the additional quantizing that entails.

And even when you have viewed a real rainbow in real time, if one (or a dozen, or a hundred thousand) raindrops within the auspices of those nominally refracting the blue end of the spectrum, happened to have dissolved in it some contaminant that caused it to refract or reflect only the red components of the spectrum, do you think that given the persistence of your vision, you would detect the resultant, brief pink hue?

Even via the analogue processes of film or slide production, and with the fastest shutter speed possible, the energies of many millions of photons will have contributed to, and been averaged to produce the hue displayed for any given point in the picture.

All the scientific models are based around infinitesimal points and pure frequency sources, but it is naive to believe that this could ever occur outside of laboratory conditions. That's why NASA go to such great lengths to try and adjust the raw data produced by their cameras on Mars, or Cassini, to produce "naturalistic" color photos. So that we might get something of an impression looking at those photos of what we would actually see were we standing where their cameras are mounted.

The colours we perceive, and the labels we attach to them, are entirely constructed within our brains. And for any hue we perceive, there are almost infinite number of mixes of frequencies of photons that will reach our eyes in a given period of time, from a given vector, that will result in us perceiving that hue.

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.
I'm sorry, but you aren't exactly making a lot of sense. While theoretically there are a lot of things that could happen for split seconds, depending on which receptors get hit, etc, the odds against them happening are incredibly high. And the odds against them continuing to happen long enough to get noticed are so high that that is effectively impossible.

To a very high degree of precision, a rainbow will cause specific pure frequencies to arrive at specific angles, with a calculable correlation between the angle and the spectrum that arrives. To a reasonable degree of precision, you will perceive a specific frequency of light as a specific colour. Most people will have the similar responses to specific frequencies of light. And there is simply no pure frequency of light that is perceived as pink.

This is pretty basic, and should be pretty simple. I have verified this in the past while looking at real rainbows, pictures of rainbows, and the output of prisms. Philosophical musings on the caprices of point sources, chance photons, and pure frequencies don't change it. Neither do howlers about why the output from cameras on various space missions needs massaging to produce natural looking photographs. (Giant hint, the various cameras used on space missions generally have frequency responses that aren't close to any of the major receptors in our eyes.)

As far as I am concerned this thread is over. Respond if you like, but I see no point in repeating myself.

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