The other day I saw a green light emitted from some source far away, and I realised that if I looked at it out of the corner of my eye I perceived it completely white. What is the explanation for this? Should this be more of a biology of the human eye question perhaps?

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    $\begingroup$ I’m voting to close this question because this is about the physiology of the eye, not physics. $\endgroup$
    – ProfRob
    Jul 23, 2020 at 7:32
  • $\begingroup$ Was it at daytime or nighttime? What was the brightness of the source? $\endgroup$
    – Ruslan
    Jul 23, 2020 at 8:02
  • $\begingroup$ @RobJeffries surely someone can give me a physical explanation so as to why the physiology of the eye works in such interesting way $\endgroup$
    – Luismi98
    Jul 23, 2020 at 10:29
  • $\begingroup$ @Ruslan it was in the evening and the source was pretty bright :) $\endgroup$
    – Luismi98
    Jul 23, 2020 at 10:30
  • $\begingroup$ As to the why, I believe evolution plays a role. $\endgroup$
    – ProfRob
    Jul 23, 2020 at 11:01

1 Answer 1


Take a look at https://xkcd.com/1080/large/. Red and green-sensing cones are mostly found in the center of your visual field. Interestingly, this is not true for blue-sensing cones. It's also not true for rods, which detect black-and-white, so the reason the light appeared white is likely that it was picked up mostly by rods.


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