Why when we click a photo in nigh sometimes our eyes show red?? I know that due to flash light reaction but it reach with which thing ??

In flash photography the light of the flash occurs too fast for the pupil to close, so much of the very bright light from the flash passes into the eye through the pupil,reflects off thefundusat the back of the eyeball and out through the pupil.The camera records this reflected light.The main cause of the red color is the ample amount of blood in the choroid which nourishes the back of the eye and is located behind the retina.
The blood in the retinal circulation is far less than in the choroid,and plays virtually no role.The eye contains several photostable pigments that all absorb in the short wavelength region,and hence contribute somewhat to the red eye effect.