What is the cause of the "red-eye" phenomenon in flash photography?
The central area of the retina (the fovea or macula, where the color
discriminating cones are) has a reflector behind it so that* light
makes two passes through the rather transparent photoreceptors. In
so doing it also passes twice through the capilary bed on the inside
of the retina, which may be responsible for the red color. In any
event it is the reflected light one sees through pupils which are
dilated in subdued light. A partial fix is achieved on modern cameras
which flash twice, the first time to stimulate contraction in the
pupils, and the second to take the picture. Reflection from the eyes
of animals, particularly nocturnal carnivores, when light is shone on
them arises from a similar cause, only for some reason the reflected
light is not red.
Leigh
*If you choose to read this as teleology that's OK.