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View Full Version : What Determines The Color of Reflected Light (From Dog's Eyes)


ralph124c
08-31-2011, 08:09 PM
My dog's eyes reflect as red light-the neighbor's dog is green.
And I have seen some dogs whose eyes reflect blue light.
Why is this?

CalMeacham
08-31-2011, 09:54 PM
It's determined by the Tapetum (properly the Tapetum Lucida), which is a reflective layer at the back of the eye that reflects light back through the retina. One thing I hate about people who explain it this way is that it leaves out why this is important or works -- it gives the retina a second chance to register light that has passed through it unabsorbed in the first pass, effectively doubling its optical length

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


I hadn't noticed any difference in reflected eye color among dog species. My cats seem to reflect yellow. People lack a tapetum, but you can get a red reflection from the blood vessels in the eye that is directed back toward a camera by the same focusing optics that make the image on the retina, thuis producing the infamous "red eye" on pictures you take with a flash.

Scarlett67
08-31-2011, 10:51 PM
I asked this question here many moons ago, and I don't recall whether I got a definitive answer. All of our dogs' eyes have always reflected red -- except for Scout, the shepherd/lab mix, whose eyes glowed bright Kryptonite green. It was spooky.

DSeid
09-01-2011, 09:23 AM
... -- it gives the retina a second chance to register light that has passed through it unabsorbed in the first pass, effectively doubling its optical length

http://en.wikipedia.org/wiki/Tapetum_lucidum ...
Not quite sure what are meaning by that effectively doubling its optical length bit. Your cite though gets it right. It increases low light sensitivity by maximizing photon capture but at the cost of some blurriness. The species that have independently evolved it tend to hunt at night.

Mooch
09-01-2011, 11:12 AM
My dog has two different colored eyes each of which reflects a different color (red and green).

Is tapetum color determined by eye color?

DSeid
09-01-2011, 12:11 PM
Click on his wiki link.Cats and dogs with blue eyes (see Eye color) may display both eyeshine and red-eye effect. Both species have a tapetum lucidum, so their pupils may display eyeshine. In flash color photographs, however, individuals with blue eyes may also display a distinctive red eyeshine. Individuals with heterochromia may display red eyeshine in the blue eye and "normal" yellow / green / blue / white eyeshine in the other eye. These include odd-eyed cats and bi-eyed dogs. The red-eye effect is independent of the eyeshine: in some photographs of individuals with a tapetum lucidum and heterochromia, the eyeshine is dim yet the pupil of the blue eye still appears red. This is most apparent when the individual is not looking into the camera, because the tapetum lucidum is far less extensive than the retina.

Zsofia
09-01-2011, 01:55 PM
As for cats, two of mine reflect the usual green and one reflects orange. They all have similar eye colors, too.

CalMeacham
09-01-2011, 02:04 PM
Not quite sure what are meaning by that effectively doubling its optical length bit.

The light passes through the same length of retina twice. That's twice the optical length. are you implying that's incorrect?

DSeid
09-01-2011, 05:10 PM
The light passes through the same length of retina twice. That's twice the optical length. are you implying that's incorrect?
Well I don't understand what it has to do with the advantage of a tapetum. The light hits the back of the eye with it's optical length, focused on the retina. Some light do not hit any receptors and are reflected, actually twice, off the tapetum in the back, to the front, and reflected back again. More chance to catch a photon but less focus. More optical length somehow implies a focusing advantage to me and that is not involved at all. In any case the extra distance the light travels is not the important part.

DSeid
09-01-2011, 09:06 PM
My confusion as well. No, it does not work that way and no increase in optical length. Any photons that miss receptors are hit the tapetum and are reflected off at a slightly different angle to possibly hit a nearby but slightly differently located receptor on its way back through the retina. The extra distance travelled is minute.