# Why do some animals' eyes glow a lot when light shines on them... **URL:** \ **Category:** Factual Questions\ **Created:** [March 13, 2019, 12:35pm UTC](https://boards.straightdope.com/t/why-do-some-animals-eyes-glow-a-lot-when-light-shines-on-them/831064 "2019-03-13T12:35:38Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Oly](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/oly/32/8255_2.png) [@Oly](https://boards.straightdope.com/u/Oly)\ **Post date:** [March 13, 2019, 12:35pm UTC](https://boards.straightdope.com/t/why-do-some-animals-eyes-glow-a-lot-when-light-shines-on-them/831064/1 "2019-03-13T12:35:38Z")
…while other animals’, including humans, don’t? Like my dog, when I shine a flashlight at him, his eyes light up like LEDs. That doesn’t happen when I shine the same flashlight at the teenager taking an unauthorized whizz in my rhododendron. ---
**Author:** ![Anaglyph](https://avatars.discourse-cdn.com/v4/letter/a/ed8c4c/32.png) [@Anaglyph](https://boards.straightdope.com/u/Anaglyph)\ **Post date:** [March 13, 2019, 12:41pm UTC](https://boards.straightdope.com/t/why-do-some-animals-eyes-glow-a-lot-when-light-shines-on-them/831064/2 "2019-03-13T12:41:54Z")
See: Tapetum lucidum ([Tapetum lucidum - Wikipedia](https://en.wikipedia.org/wiki/Tapetum_lucidum)). A light reflecting layer behind the retina reflects the light, thereby improving the night vision of animals having this feature. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [March 13, 2019, 1:29pm UTC](https://boards.straightdope.com/t/why-do-some-animals-eyes-glow-a-lot-when-light-shines-on-them/831064/3 "2019-03-13T13:29:02Z")
> [@Anaglyph](#): > > See: Tapetum lucidum ([Tapetum lucidum - Wikipedia](https://en.wikipedia.org/wiki/Tapetum_lucidum)). A light reflecting layer behind the retina reflects the light, thereby improving the night vision of animals having this feature. This. When the incident light first hits the retina, some of it is intercepted by photoreceptors and the rest of it passes right through. The tapetum reflects it back through the retina one more time, giving the photoreceptors therein another opportunity to intercept additional photons. It’s similar to increasing the exposure time on a camera, in that more light gets detected. Even on the second pass though, not all of the light gets intercepted; a good deal of it gets blasted forward out of the eye again, back toward the source (you and your flashlight). It’s not terribly different from shining a light toward [retroreflective sheeting.](https://en.wikipedia.org/wiki/Retroreflective_sheeting) ---
**Author:** ![Paul\_in\_Qatar](https://avatars.discourse-cdn.com/v4/letter/p/ccd318/32.png) [@Paul\_in\_Qatar](https://boards.straightdope.com/u/Paul_in_Qatar)\ **Post date:** [March 13, 2019, 2:05pm UTC](https://boards.straightdope.com/t/why-do-some-animals-eyes-glow-a-lot-when-light-shines-on-them/831064/4 "2019-03-13T14:05:39Z")
Animal’s eyes glow red. Our eyes glow white. If you take a picture of someone and their eyes glow red, get them to a doctor. I am not kidding. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [March 13, 2019, 2:08pm UTC](https://boards.straightdope.com/t/why-do-some-animals-eyes-glow-a-lot-when-light-shines-on-them/831064/5 "2019-03-13T14:08:12Z")
> [@Machine\_Elf](#): > > This. When the incident light first hits the retina, some of it is intercepted by photoreceptors and the rest of it passes right through. The tapetum reflects it back through the retina one more time, giving the photoreceptors therein another opportunity to intercept additional photons. It’s similar to increasing the exposure time on a camera, in that more light gets detected. Even on the second pass though, not all of the light gets intercepted; a good deal of it gets blasted forward out of the eye again, back toward the source (you and your flashlight). > > It’s not terribly different from shining a light toward [retroreflective sheeting.](https://en.wikipedia.org/wiki/Retroreflective_sheeting) This is accurate for most animals. But for cats it’s because they are the spawn of the devil. ---
**Author:** ![bump](https://avatars.discourse-cdn.com/v4/letter/b/7c8e57/32.png) [@bump](https://boards.straightdope.com/u/bump)\ **Post date:** [March 13, 2019, 2:12pm UTC](https://boards.straightdope.com/t/why-do-some-animals-eyes-glow-a-lot-when-light-shines-on-them/831064/6 "2019-03-13T14:12:36Z")
> [@Paul\_in\_Qatar](#): > > Animal’s eyes glow red. Our eyes glow white. If you take a picture of someone and their eyes glow red, get them to a doctor. I am not kidding. Or an exorcist… ---
**Author:** ![Walken\_After\_Midnight](https://avatars.discourse-cdn.com/v4/letter/w/b487fb/32.png) [@Walken\_After\_Midnight](https://boards.straightdope.com/u/Walken_After_Midnight)\ **Post date:** [March 13, 2019, 2:18pm UTC](https://boards.straightdope.com/t/why-do-some-animals-eyes-glow-a-lot-when-light-shines-on-them/831064/7 "2019-03-13T14:18:28Z")
> [@Paul\_in\_Qatar](#): > > Our eyes glow white. If you take a picture of someone and their eyes glow red, get them to a doctor. I am not kidding. Isn’t red normal, and white, yellow or black [abnormal](https://www.aao.org/eye-health/tips-prevention/diagnosing-children-from-photographs)? ---
**Author:** ![markn\_1](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@markn\_1](https://boards.straightdope.com/u/markn_1)\ **Post date:** [March 13, 2019, 2:20pm UTC](https://boards.straightdope.com/t/why-do-some-animals-eyes-glow-a-lot-when-light-shines-on-them/831064/8 "2019-03-13T14:20:35Z")
> [@Paul\_in\_Qatar](#): > > Animal’s eyes glow red. Our eyes glow white. If you take a picture of someone and their eyes glow red, get them to a doctor. I am not kidding. Then why is it called the [red eye effect](https://en.wikipedia.org/wiki/Red-eye_effect) in humans? ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [March 13, 2019, 2:22pm UTC](https://boards.straightdope.com/t/why-do-some-animals-eyes-glow-a-lot-when-light-shines-on-them/831064/9 "2019-03-13T14:22:15Z")
> [@Paul\_in\_Qatar](#): > > Animal’s eyes glow red. Our eyes glow white. If you take a picture of someone and their eyes glow red, get them to a doctor. I am not kidding. Have you ever heard of redeye in pictures 😕 ---
**Author:** ![bardos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bardos/32/1069_2.png) [@bardos](https://boards.straightdope.com/u/bardos)\ **Post date:** [March 13, 2019, 5:07pm UTC](https://boards.straightdope.com/t/why-do-some-animals-eyes-glow-a-lot-when-light-shines-on-them/831064/10 "2019-03-13T17:07:49Z")
> [@markn\_1](#): > > Then why is it called the [red eye effect](https://en.wikipedia.org/wiki/Red-eye_effect) in humans? I have noticed that certain airlines use this on some of their flights ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [March 13, 2019, 6:37pm UTC](https://boards.straightdope.com/t/why-do-some-animals-eyes-glow-a-lot-when-light-shines-on-them/831064/11 "2019-03-13T18:37:09Z")
Even without the tapetum lucidum, the eye acts as a crude retro-reflector (reflecting light back towards the light source). Because the light is focused at the retina, and some of it is scattered/reflected, and this is re-focused by the eye and sent back the way it came. Which is why red-eye only happens in photos taken with a flash - because the light source (flash) is very close to the camera lens. ---
**Author:** ![Kearsen1](https://avatars.discourse-cdn.com/v4/letter/k/3d9bf3/32.png) [@Kearsen1](https://boards.straightdope.com/u/Kearsen1)\ **Post date:** [March 13, 2019, 6:43pm UTC](https://boards.straightdope.com/t/why-do-some-animals-eyes-glow-a-lot-when-light-shines-on-them/831064/12 "2019-03-13T18:43:58Z")
> [@Paul\_in\_Qatar](#): > > Animal’s eyes glow red. Our eyes glow white. If you take a picture of someone and their eyes glow red, get them to a doctor. I am not kidding. Negatory, predators eyes glow red (including humans), non-predatory animals are white ---
**Author:** ![JWT\_Kottekoe](https://avatars.discourse-cdn.com/v4/letter/j/3ec8ea/32.png) [@JWT\_Kottekoe](https://boards.straightdope.com/u/JWT_Kottekoe)\ **Post date:** [March 14, 2019, 1:51am UTC](https://boards.straightdope.com/t/why-do-some-animals-eyes-glow-a-lot-when-light-shines-on-them/831064/13 "2019-03-14T01:51:42Z")
Photodiodes are sometimes made this way. A thinner layer of light absorbing material is used to reduce the transit time of electrons and holes. But this reduces the responsivity, since not all the light is absorbed. By bouncing the light back for one more pass you can gain back some of the lost quantum efficiency, without slowing the device down. If the light is monochromatic, you can carry this a step further by bouncing the light many times. This is a resonant photodiode. ---
**Author:** ![brossa](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/brossa/32/3040_2.png) [@brossa](https://boards.straightdope.com/u/brossa)\ **Post date:** [March 14, 2019, 4:01am UTC](https://boards.straightdope.com/t/why-do-some-animals-eyes-glow-a-lot-when-light-shines-on-them/831064/14 "2019-03-14T04:01:43Z")
> [@Paul\_in\_Qatar](#): > > Animal’s eyes glow red. Our eyes glow white. If you take a picture of someone and their eyes glow red, get them to a doctor. I am not kidding. This is backwards; red eyes in humans in flash photos is normal. It’s white or some other color that can represent pathology. [Leukocoria](https://en.wikipedia.org/wiki/Leukocoria) or white-eye reflex is a sign of retinoblastoma, for example. Coat’s disease can show up as xanthocoria, or yellow pupil reflex. Cataracts can result in bluish or grey pupils. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [March 14, 2019, 4:44am UTC](https://boards.straightdope.com/t/why-do-some-animals-eyes-glow-a-lot-when-light-shines-on-them/831064/15 "2019-03-14T04:44:56Z")
> [@Paul\_in\_Qatar](#): > > Animal’s eyes glow red. Our eyes glow white. If you take a picture of someone and their eyes glow red, get them to a doctor. I am not kidding. > [@Kearsen1](#): > > Negatory, > > predators eyes glow red (including humans), non-predatory animals are white Wrong. The eyes of many animals that are active at night glow white, including [cats,](https://farm4.staticflickr.com/3557/3349807586_fcc3375733.jpg) [bears,](https://outdoorhub-res.cloudinary.com/image/fetch/f_auto,q_auto:low,w_1000,h_583,dpr_auto/https://www.outdoorhub.com/wp-content/uploads/sites/2/2015/03/outdoorhub-everything-ever-wanted-know-black-bear-eyesight-2015-03-31_16-39-47.jpg) [cows](https://sadanduseless.b-cdn.net/wp-content/uploads/2019/01/cows-in-the-dark4.jpg), and [deer,](https://www.northcountrypublicradio.org/news/images/deerinheadlights.jpg) regardless of whether they are predators or not. (The eyes of some nocturnal animals may glow red or other colors.) The main factor is the presence of the tapetum lucidum, as explained. Human eyes do not glow white, because we lack a tapetum lucidum, since we are mostly diurnal. In fact, humans show little eye shine at all at night under normal conditions. However, bright flashes used in photography produce the red eye effect by reflection off the blood rich choriod layer behind the retina. ---
**Author:** ![bobmarleykingg](https://avatars.discourse-cdn.com/v4/letter/b/ee7513/32.png) [@bobmarleykingg](https://boards.straightdope.com/u/bobmarleykingg)\ **Post date:** [March 15, 2019, 3:22pm UTC](https://boards.straightdope.com/t/why-do-some-animals-eyes-glow-a-lot-when-light-shines-on-them/831064/16 "2019-03-15T15:22:23Z")
The glow of red human eyes is a little like the glow of the moon. It’s the reflected light of a far greater light source. The flash of a camera travels through the pupil and hits the back of the eye – the retina. The retina reflects the light back towards the camera, but it does so just a little differently than it came in. Like most of the insides of our bodies, the retina is an icky red mess, with blood vessels criss-crossing it. Those blood vessels color the light that shines back red. We literally dye the light red with our blood. ---
**Author:** ![Paul\_in\_Qatar](https://avatars.discourse-cdn.com/v4/letter/p/ccd318/32.png) [@Paul\_in\_Qatar](https://boards.straightdope.com/u/Paul_in_Qatar)\ **Post date:** [March 15, 2019, 4:39pm UTC](https://boards.straightdope.com/t/why-do-some-animals-eyes-glow-a-lot-when-light-shines-on-them/831064/17 "2019-03-15T16:39:37Z")
> [@brossa](#): > > This is backwards; red eyes in humans in flash photos is normal. It’s white or some other color that can represent pathology. [Leukocoria](https://en.wikipedia.org/wiki/Leukocoria) or white-eye reflex is a sign of retinoblastoma, for example. Coat’s disease can show up as xanthocoria, or yellow pupil reflex. Cataracts can result in bluish or grey pupils. Yes. I got it backwards. The Editor regrets the error. ---
**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\ **Post date:** [March 15, 2019, 11:17pm UTC](https://boards.straightdope.com/t/why-do-some-animals-eyes-glow-a-lot-when-light-shines-on-them/831064/18 "2019-03-15T23:17:22Z")
And technically, nobodies eyes **glow** , because they don’t produce any light. They only _reflect_ light that is coming from another source. ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [March 16, 2019, 1:13am UTC](https://boards.straightdope.com/t/why-do-some-animals-eyes-glow-a-lot-when-light-shines-on-them/831064/19 "2019-03-16T01:13:04Z")
The eyes of flying squirrels [glow rainbow](https://imgur.com/a/f5XXIbv). ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [March 16, 2019, 2:26am UTC](https://boards.straightdope.com/t/why-do-some-animals-eyes-glow-a-lot-when-light-shines-on-them/831064/20 "2019-03-16T02:26:33Z")
> [@Tim\_T-Bonham.net](#): > > And technically, nobodies eyes **glow** , because they don’t produce any light. > They only _reflect_ light that is coming from another source. To nitpick your nitpick, “glow” means to shine with a steady light. It doesn’t necessarily mean that something is emitting light on its own. [Next page](https://boards.straightdope.com/t/why-do-some-animals-eyes-glow-a-lot-when-light-shines-on-them/831064.md?page=2)