Why do dogs and cats have red eyes in photographs?

Answer 1

Here's a great video on why our eyes shine red in photos:

https://www.youtube.com/watch?v=FRN1RpCnKLk

Once you have that down, then you can think about the tapetum lucidum, which is an extra reflective layer behind the retina in dogs, cats, and other animals with better night vision than us. That layer in most dogs causes a different color glow in dogs' eyes (sometimes green) when they are looking directly at the lens when you take a photo of them. The tapetum changes over the course of an animal's life, causing the color to be different in animals of different ages when you see this effect. Also, some breeds of cats and dogs have tapeta that differ in ways (like pigment) that cause them to shine different colors, including red, which often happens when the tapetum lacks enough pigment to block the reflection of light off the blood vessels in the retina.

Answer 2

Dogs and cats have an extra part in the anatomy of their eyes, called the tapetum lucidum. It's a reflective layer on the retina of carnivorous nocturnal mammals that helps with night vision. The reflected light bounces back and gets a second chance for absorption by light sensitive rods and cones. Their final image is slightly blurry but much brighter. When we shine a light on their eyes that like reflects in an iridescent way, that often looks red. The color red is either caused by the pigment found in the blood of the eye, called hemoglobin, like why if you put a flashlight under your finger the light is red. It could also be because shorter wave lengthens of light are attenuated, like why we have red sunsets.