Why in flash light photo of eyeball become red?

look at the eyes 

After longing research finally, I got this reason why it is so.

It is the actually same reason as back-light of bicycle works at night means the same principle as retro reflector glass is working. [ link for details how retro-reflector works ]

                                                          ( retro-reflector glass)

 As our eyeball is spherical in shape the light is reflected back to the source itself means if we capture a photo by mobile phone as the flashlight is configured side by side to the camera, the light will immediately back to the camera itself, for that reason under the flashlight at night eyeball become brighter like a Dracula! 

                                        

[schematic diagram of an EYE ball] 

Now question arise why reddish in colour?

so again if we back to our physics class we can find the answer, we know when a beam of light passes through a lens it will get separated as VIBGYOR, of which red colour has less diffraction means long way can travel without deflecting more. so that's why when the reflected light back to the towards source passes through the lens, it gets separated as result camera perceived mostly red light.

Also, another reason for reddish colour is our uncountable red blood cell present inside the eyes.

     

But sometimes it reflected bluish light also it totally depends upon incidence angle of light falling on the lens, size of the lens(as the lens are flexible) at that moment. As I have shown below in this picture.

look at those figure different eye colour at a different angle

HOPE YOU LEARN SOMETHING NEW TODAY!