Why does red eye appear when taking pictures with flash?

First of all, we need a comprehensive understanding of light. The light we see in our daily life is only visible light. In addition to visible light that we can see with the naked eye, there are other substances in nature that we can hardly see with the naked eye, such as gamma rays, X rays, ultraviolet rays, infrared rays, microwaves and electromagnetic waves. Of course, visible light is undoubtedly the most typical kind of light, which has the characteristics of various electromagnetic radiation light waves.

People who are photographed by flash can see red in their eyes, which can be reflected from many other substances besides their eyes. It turns out that when light enters the eyes from the retina, the retina will perceive it and then transmit the image to the brain through the optic nerve. Pupil dilation will determine how much visible light the retina can receive. Whether the pupil can contract is affected by the external light intensity. For example, on a sunny day, the pupil will naturally contract, reducing the entry of light and avoiding the eyes being stabbed by too strong light; In the dark, the pupils dilate, allowing more visible light to enter the eyes. There are many blood vessels in choroid between retina and sclera, which can provide oxygen and nutrients for retina.

If people's pupils don't have enough time to contract when taking photos with flashlights, a lot of visible light emitted by these flashlights will reflect back from the fundus (the inner surface of the eyes). The reflected light will show choroidal blood vessels, and the reflected light will be red when reflected on the lens. When people take pictures, the angle of light entering the eyes is the same as the angle of reflection, so the closer the flashlight is to the lens, the greater the possibility of reflected light hitting the lens. In a dark environment, people's pupils will try their best to dilate, and the glasses in the photo are likely to appear red.

In addition to the above two reasons, the color of the eyeball itself, people's skin color and age will all affect whether the glasses appear red in the photos taken by flashlight. These factors also apply to animals. For example, when we shine a flashlight on a pig's eyes, its eyes will reflect red light, while when we shine a flashlight on a dog's eyes, it will emit blue light or green light.

In short, when you see people's eyes glowing red in a photo, don't think it's a strange phenomenon. In fact, this phenomenon has scientific basis.