Objects appear red in color because they reflect red light.
In the photo, light from a camera’s flash entered the pupils of the boy’s eyes. This light hit the back of each eye where some of the light was absorbed and some of the light was scattered back toward the camera.
The back of the eye contains many blood vessels, thus much of the scattered light is red.
How can a double flash camera prevent red eye?
The light flash causes the pupil to contract (get smaller). With a smaller pupil less light will enter the eye, and thus less red light will be scattered out of the pupil. The problem is that light travels at such a fast speed that the scattered red light has already left the eye and entered the camera before the pupil contracts.
Camera’s with two flashes solve the red eye problem.
1. Flash #1: This flash of light causes the eye pupils to contract, but the camera lens remains closed.
2. Flash #2: This flash corresponds with the opening of the camera’s lens. While there may be a small amount of scattered red light from the eyes, it is generally not noticeable.