Why do red eye appear on pictures?

Red eyes appear in photographs due to the reflection of light from the camera's flash off the blood vessels in the retina. When the flash fires, it can illuminate the eyes before the pupils have a chance to constrict in response, resulting in the red appearance. This effect is more pronounced in low-light conditions where the pupils are dilated, allowing more light to enter. Using red-eye reduction features or adjusting lighting can help minimize this effect in photos.