Light directly affects the eyeball, passing through the pupil to the rear blood vessel.
The reason why the eyes are red is probably because you stare at the camera's lens . You can eliminate the "red eye" effect in a simple way that is to avoid staring at the lens but just looking at a nearby light bulb before taking a picture.
To reduce the "red-eye" effect, most modern cameras often flash the front flash before the flash officially blinks, so you have enough time to squint your eyes and adapt to it.
The intensity of light reflection of each person is different, depending on the size, age and eye color of each individual. People with bright skin and blue or green eyes are less likely to be melanin (pigment-forming epidermal cells), which can often suffer from "red-eye" effects when taking pictures.
High-quality effects often appear in the eyes of humans and animal eyes, especially in the eyes of nocturnal animals - the retina has a special reflective layer called the tapetum lucidum mirror that acts as a mirror placed behind the eyes. This tissue helps animals see things better at night with actions like reflectors (retroreflector), taking reflected light and directly reflecting it back to its original direction.