The main reason why dog ​​eye beads reflect red light is because there is a reflective film in their eyes. This reflective film is composed of special reflective cells. These reflective cells can reflect light so that the light entering the eye can be reflected back. When the light hits the dog's eyes, this reflective layer will reflect red light, thus forming the red light we see.

This reflective film helps dogs see better in the dark because it strengthens the eyes of light. So, when we use flash to take photos of dogs, their eyes will appear red.