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Anonymous Poster

# Understanding Cones (Eye) and Light

02/12/2009 6:51 PM

I would appreciate clarity on something bugging me for long period. About eyes. Can anyone explain please (different books and websites use diferent ways to explain and seem to contradict) which way do the cones work?

There are diferent ones for reds, blues and green and they response which way:

if red wave enters eye, only red-cones response (y-n?). So when you see a red clothing, you see red because only the red wave reflect off it, all other waves absorbed by the pigments in garment (y-n?).

Now, are not red waves nearest infra-red and have best penetrating power (y-n?)

So why when deep underwater red colors dissappear first or not true? Is not plenty of red waves down to reflect off?! What is missing. Thank you for help.

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The Engineer

Join Date: Feb 2005
Location: Albany, New York
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#1

### Re: Problem understanding cones (eye) and light

02/12/2009 9:50 PM

Hi guest,

You should sign in and become a member, you'll find a lot of answers here, some may even be good.

"if red wave enters eye, only red-cones response (y-n?)No. So when you see a red clothing, you see red because only the red wave reflect off it, all other waves absorbed by the pigments in garment (y-n?)Basically, Yes."

Actually all the cones respond to different degrees. That's because in nature there never is just red light or blue light, there are spectrum with peaks at red or blue. And the three cone types in the eye don't just detect red or blue or green, they detect spectrum with peaks at these colors, please see the spectrum for the three cones below:

And here is the spectrum of various light sources:

Which produce light that is reflected or are absorbed by objects. Then the reflected light from the surface is a spectrum (which can depend on the light spectrum hitting it). For instance:

Which then is absorbed by the color cones in the eyes to different degrees base on their detection spectrum. Since the different cones react to the same light differently, your brain can "triangulate" the color. Hopefully that makes sense to you.

Here is a great article from wikipedia providing additional information.

http://en.wikipedia.org/wiki/Trichromatic_color_vision

"Now, are not red waves nearest infra-red and have best penetrating power (y-n?)Sometimes" -

Penetrating power depends on the what you're trying to shine light through. For instance some glass is transparent to visible light but reflects infrared (its how greenhouses work). On the other hand, zinc selenide is transparent for red and a lot of IR but not for the rest of visible (which is why it's reddish orange, see below):

So why when deep underwater red colors dissappear first or not true? Is not plenty of red waves down to reflect off?! What is missing.

Ok, to answer that question, lets look at Water's Absorption Spectrum:

Hopefully you can see that blue is absorbed less than red from the chart above. In water, blue light has more penetrating power than red. So imagine shining a white light onto water. White light has all the colors in it in a spectrum. Now, as the light travels down into the water, the red in it is absorbed quicker than the blue, so if you go down deep enough, all you have left is blue light.

Deep sea animals actually exploit this with red coloring. You see, if you shine white light on a red animal like a crab, they appear red because the red light in the white light is reflected. However, as we have just seen, red light doesn't penetrate far into water, meaning only blue light gets very far. When you shine blue light on a red animal it looks....black, of course, because nothing is reflected (or very little). It's camoflage!

Here's an interesting link that talks about the red color of ocean animals as camoflage:

http://oceanexplorer.noaa.gov/explorations/02hudson/background/edu/media/hc_bright_red.pdf

Roger

Anonymous Poster
#2

### Re: Problem understanding cones (eye) and light

02/13/2009 7:20 AM

Thanking you SO much Mr Pink - you put 2 and 2 together VERY nicely! Well done. I am most apprecitive for your kind answer.

Ray

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#3

### Re: Problem understanding cones (eye) and light

02/13/2009 2:12 PM

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A little knowledge is a dangerous thing - Googling is far worse!
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#4

### Re: Problem understanding cones (eye) and light

02/13/2009 11:23 PM

Hello Mr Pink,

A very good answer sir. Hence a GA!

Take car....................................

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#6

### Re: Problem understanding cones (eye) and light

02/13/2009 11:38 PM

GA. Really a good piece. If you don't mind I'd like to save a copy of this. I have some clients who could really use this information to understand the limits of human color vision.

Its amazing that we can see so much with what is in many ways (at least from a technical point of view) grossly inferior equipment. The optics of the eye are on a par with children's toys, and as the graph of the three cones' response curves show, the detector design is less than optimal. But out of that fairly poor data, the various feedback circuits in the retina, the optic nerve and the brain can tease out a staggering pile of conclusions regarding the visible world around us.

John

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#7

### Re: Problem understanding cones (eye) and light

02/14/2009 1:33 PM

Hi,

the detector design is not so bad but we suffer from a heritage of evolution.

During the very first time of mammals - estimated 250 million years ago - our early ancestors lived as nocturnal animals - no necessity of color vision.

They lost the magnificent eyes of earlier developments - until now surviving in birds, reptiles and fish.

These have 4 independent color sensors, ultraviolet the fourth.

On top of each sensor cell a tiny spherical droplet of oil: colored for spectral filtering and focusing onto the light sensitive area to boost sensitivity.

Then for 200 or more million years mammals had only two different color sensors as now in most of them.

Only in early primates a mutation doubled the red-sensitive types not only in number but also in genetics. One of these sets of genetic information subsequently shifted the center of spectral sensitivity towards green.

In this doubling of genes remained some flaws so a few of us have a red-green blindness.

We will never see the magnificent patterns of ultraviolet coloring of flowers except in UV-photography.

So the "visible" world is very different depending which eyes are looking and we humans have only a fair and incomplete set of what is existing in other species.

RHABE

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Posts: 5907
#5

### Re: Understanding Cones (Eye) and Light

02/13/2009 11:29 PM

this may help too

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