I am posing this question
on behalf of one cleverer than I.
He has not yet learned to type,
so here his words, as best I could transcribe.
We submit to your wisdom
hoping you’ll help answer: why, why, why?
Dear Cecil and Cecil’s friends,
Reuben was cycling past the sun very fast and there was a wooden fence in front of the sun and when Reuben looked at the sun through the fence, he saw bright flashes of red and green light in the air (because the fence was in the way; it was flashing).
Why does that happen please?
Yours very sincerely,
Reuben Gabbay (age 5 1/2)
My first guess is that it’s related to the Benham’s disk effect.
But given that Benham’s disk isn’t really understood, either, that’s not much of a help.
Oh, and I love your username.
I see something similar if I am driving when the sun is low and shines through a row of trees. It is extremely distracting to me to the point I wonder about it triggering epilepsy. I always cover the side window with my hand.
Reuben gets credit for observing this, but when I duplicated his experiment I saw the same red and green flashes. They were dazzling and surprisingly vivid.
> My first guess is that it’s related to the Benham’s disk effect.
I tested Benham’s disk and saw shades of yellow (not flashes of red and green). To be fair, conditions were different: computer screen vs the sun, so it could be the same mechanism manifesting itself differently.
> Oh, and I love your username.
>blushes< Thank you.
> I see something similar if I am driving when the sun is low and shines through a row of trees.
Yes, it sounds like very similar circumstances. Thank you; it’s good to hear from somebody else on this.
My theory, for what it’s worth, is this: the red flashes were scattered light bouncing off the blood in the eye, which was so strong that it overloaded the red cones, so that the next incoming appeared green. Because the sun was low, there was little blue light around to confuse things, so we saw flashes of red and green.
Comments welcome.
Not intending to hijack but can we find any statistical analysis of setting-sunlight through staggered trees triggering epilepsy? I am not epileptic but it’s so incredibly jarring that I always think “this MUST be a trigger, surely!”
I get it also but I rarely see the green. Mine is typically black and red. I usually just close the eye on that side and drive half blind.
Hmm…but why would red light get reflected/scattered in the eye in this situation and not in others? In other words, doesn’t your theory predict that a camera flash would also produce red/green sub-flashes?
I’m speculating, but a slatted fence like you described could act as a set of diffraction slits, especially if the rider wasn’t riding exactly parallel to the fence and the sun’s rays weren’t exactly perpendicular to the fence.
That would mean that the apparent width of each slit in the fence would increase and then decrease as the rider passed—in other words, perspective could narrow each slot’s apparent width so much that it diffracts some of the light passing through it.
This is easy enough to test—the next time you observe these red/green flashes through the fence, get a white piece of paper and move it around. If this theory is right, you should be able to see the paper reflect red and green light as you move it slowly in the same direction you were riding when you saw the flashes.
If you decide to conduct that experiment, please share the results!
ETA: Now that I think about it, the intensity of the red/green flashes could be explained by constructive interference in the diffracted light of multiple slits. If so, it would make the color on the paper even more prominent.
> This is easy enough to test—the next time you observe these red/green flashes through the fence, get a white piece of paper and move it around. If this theory is right, you should be able to see the paper reflect red and green light as you move it slowly in the same direction you were riding when you saw the flashes.
This is an excellent suggestion, thank you.
No red/green fringing was visible on the ground (plain concrete slabs) in front of the fence, but your suggestion to hold up a white piece of paper at eye level is sound.
The circumstances will be hard to replicate, but if I can I will.
> Hmm…but why would red light get reflected/scattered in the eye in this situation and not in others? In other words, doesn’t your theory predict that a camera flash would also produce red/green sub-flashes?
Also a good question. You may be right, but the analogy is not perfect.
What generated the illusion was a dazzlingly bright flashing yellow light entering the eye from the side, possibly at some specific frequency that is slow enough to overload the cones, but quick enough to be back before they’re fully recovered.
A strobing camera flash (as a red-eye prevention strobe) is a blue-white light, usually entering the eye square on, flashing perhaps rather quicker, and is bright but not necessarily as overwhelming a flood of photons as put out by a late afternoon sun hanging on the horizon.