# What Determines The Color of Reflected Light (From Dog's Eyes) **URL:** \ **Category:** Factual Questions\ **Created:** [September 1, 2011, 1:09am UTC](https://boards.straightdope.com/t/what-determines-the-color-of-reflected-light-from-dogs-eyes/594808 "2011-09-01T01:09:14Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![ralph124c](https://avatars.discourse-cdn.com/v4/letter/r/8797f3/32.png) [@ralph124c](https://boards.straightdope.com/u/ralph124c)\ **Post date:** [September 1, 2011, 1:09am UTC](https://boards.straightdope.com/t/what-determines-the-color-of-reflected-light-from-dogs-eyes/594808/1 "2011-09-01T01:09:14Z")
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? ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [September 1, 2011, 2:54am UTC](https://boards.straightdope.com/t/what-determines-the-color-of-reflected-light-from-dogs-eyes/594808/2 "2011-09-01T02:54:53Z")
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 > **[Tapetum lucidum](https://en.wikipedia.org/wiki/Tapetum_lucidum)** > > The tapetum lucidum (Latin for 'bright tapestry, coverlet'; /təˈpiːtəm ˈluːsɪdəm/ tə-PEE-təm LOO-sih-dəm; pl. tapeta lucida) is a layer of tissue in the eye of many vertebrates and some other animals. Lying immediately behind the retina, it is a retroreflector. It reflects visible light back through the retina, increasing the light available to the photoreceptors (although slightly blurring the image). The tapetum lucidum contributes to the superior night vision of some animals. Many of these Si... 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. ---
**Author:** ![Scarlett67](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/scarlett67/32/7387_2.png) [@Scarlett67](https://boards.straightdope.com/u/Scarlett67)\ **Post date:** [September 1, 2011, 3:51am UTC](https://boards.straightdope.com/t/what-determines-the-color-of-reflected-light-from-dogs-eyes/594808/3 "2011-09-01T03:51:51Z")
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. ---
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [September 1, 2011, 2:23pm UTC](https://boards.straightdope.com/t/what-determines-the-color-of-reflected-light-from-dogs-eyes/594808/4 "2011-09-01T14:23:00Z")
> [@CalMeacham](#): > > … – 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 > > [Tapetum lucidum - Wikipedia](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. ---
**Author:** ![Mooch](https://avatars.discourse-cdn.com/v4/letter/m/7bcc69/32.png) [@Mooch](https://boards.straightdope.com/u/Mooch)\ **Post date:** [September 1, 2011, 4:12pm UTC](https://boards.straightdope.com/t/what-determines-the-color-of-reflected-light-from-dogs-eyes/594808/5 "2011-09-01T16:12:12Z")
My dog has two different colored eyes each of which reflects a different color (red and green). Is tapetum color determined by eye color? ---
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [September 1, 2011, 5:11pm UTC](https://boards.straightdope.com/t/what-determines-the-color-of-reflected-light-from-dogs-eyes/594808/6 "2011-09-01T17:11:08Z")
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. ---
**Author:** ![Zsofia](https://avatars.discourse-cdn.com/v4/letter/z/7bcc69/32.png) [@Zsofia](https://boards.straightdope.com/u/Zsofia)\ **Post date:** [September 1, 2011, 6:55pm UTC](https://boards.straightdope.com/t/what-determines-the-color-of-reflected-light-from-dogs-eyes/594808/7 "2011-09-01T18:55:11Z")
As for cats, two of mine reflect the usual green and one reflects orange. They all have similar eye colors, too. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [September 1, 2011, 7:04pm UTC](https://boards.straightdope.com/t/what-determines-the-color-of-reflected-light-from-dogs-eyes/594808/8 "2011-09-01T19:04:21Z")
> [@](#): > > 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? ---
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [September 1, 2011, 10:10pm UTC](https://boards.straightdope.com/t/what-determines-the-color-of-reflected-light-from-dogs-eyes/594808/9 "2011-09-01T22:10:01Z")
> [@CalMeacham](#): > > 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. ---
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [September 2, 2011, 2:06am UTC](https://boards.straightdope.com/t/what-determines-the-color-of-reflected-light-from-dogs-eyes/594808/10 "2011-09-02T02:06:18Z")
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.