Although it looks magical and somewhat sinister, there’s a reasonable scientific explanation for it. So what causes your blue (or brown or hazel) eyes to turn red in certain photographs?

The red-eye effect happens when flash pictures are taken in low-light situations. When the flash is located close to the camera lens (as it is in most cameras), you may be looking almost directly at the flash when you look at the camera lens.

The red-eye effect results when the light of the flash occurs too quickly for the pupil of the eye to close. The bright light of the flash passes into the eye through the pupil, reflects off the retina at the back of the eyeball and returns back out through the pupil.

The camera records this reflected light. It appears red because of the amount of blood located in the back of the eyeball.

The red-eye effect tends to be stronger in people with light eye colors, light hair colors and fair complexions. Scientists believe this is because these people have less melanin in their eyes. Melanin helps to reduce the red-eye effect to some extent in people with darker eyes, hair colors and complexions.

If you have pets, you may have noticed that the red-eye effect can be quite different in animals. Animals have a special reflective layer at the back of their eyes called the “tapetum.”

The tapetum helps to enhance night vision. Depending on the animal, the tapetum can cause a blue, green or yellow effect on the eyes.