Camera flashes do not make your eyes turn red. The inside of your eyes are always red. The bright light of the camera flash just makes the color more obvious. Your eye is essentially a hollow ball filled with clear fluid. The hole at the front of your eye, the pupil, lets light into the hollow space inside the eye. The light passes through this space and then strikes the inner back surface of the eye, known as the retina. The retina is packed with cells that detect the light, change it to electrical signals, and pass the signals on to the brain where the pattern of light is experienced as a visual image. The approximately 100 million light-sensitive cells on the human retina give us an amazing visual resolution, but they also require a prodigious amount of blood to keep them going. This blood is what gives the retina its red color.