"Things You Wouldn't Know If We Didn't Blog Intermittently."
18 March 2026
Interesting photograph
The underside of a brick bridge illluminated by light reflected off water, with perhaps something else casting a shadow - I haven't figured that part out. No info re location at the Facebook source.
I think the brightest part is in direct sunlight, the main half-bright area under the bridge is in ambient daylight with highlights from water reflection, and the dark area is in full shadow. Maybe. Other examples at https://www.instagram.com/p/DU57Tm4AS11/?img_index=5 and at https://www.petersfieldphotographicsociety.org.uk/galleries/anthony-beggs/#gallery[photonic-flickr-set-1]/51502343918/
The top edge of the bright triangle continues from the brick onto the water surface, and there is no direct sunlight "behind" it to reflect back up on the brick.
After long consideration, I think the answer is that the wall curves BACK from the middle point where the two triangles meet. Thus the sun (located far to the right) lights up the "right" "bright" triangle, leaving the left dark triangle without any direct sun. The water of course reflects a duplication.
I suspect there may be other photos of this bridge online that would explain this optical curiosity.
I think it may be the interface between the umbra and the antumbra in this diagram https://www.coopertoons.com/education/aristarchus/differentlengthumbras.svg Since the shadow is created by an arch and not a sphere would explain the reversal to what you see here and why one is brighter.
How about this: The two halves of the bright and dark triangles are inversely related. The top half of the dark one is the shadow cast by the bridge; its lower half is the shadow's watery reflection. As to the bright triangle, while the sun shines directly on both its halves, the water below reflects its light to the bricks above, which are already bathed in direct sunlight.
I think the brightest part is in direct sunlight, the main half-bright area under the bridge is in ambient daylight with highlights from water reflection, and the dark area is in full shadow. Maybe. Other examples at https://www.instagram.com/p/DU57Tm4AS11/?img_index=5 and at https://www.petersfieldphotographicsociety.org.uk/galleries/anthony-beggs/#gallery[photonic-flickr-set-1]/51502343918/
ReplyDeleteGhuh, interesting! (And beautiful). The community over at reddit.com/r/atoptics might like this one.
ReplyDeleteBright triangle is direct sunlight, ripply is reflection from the water, dark triangle is neither.
ReplyDeleteDark triangle is shadow / shaded area?
DeleteYes, obviously. But I'm trying to parse out what straight edge created that portion of the shadow. It's not important; I just find it curious.
DeleteThe top edge of the bright triangle continues from the brick onto the water surface, and there is no direct sunlight "behind" it to reflect back up on the brick.
DeleteCodex: If I reduce the image I see a metal rod/brick ? sticking up in the middle could be artifact or distortion as well.
ReplyDeleteAfter long consideration, I think the answer is that the wall curves BACK from the middle point where the two triangles meet. Thus the sun (located far to the right) lights up the "right" "bright" triangle, leaving the left dark triangle without any direct sun. The water of course reflects a duplication.
ReplyDeleteI suspect there may be other photos of this bridge online that would explain this optical curiosity.
I think it may be the interface between the umbra and the antumbra in this diagram https://www.coopertoons.com/education/aristarchus/differentlengthumbras.svg Since the shadow is created by an arch and not a sphere would explain the reversal to what you see here and why one is brighter.
ReplyDeleteAt first glance, it looked like a seashell of some sort sitting on a dark wooden table. :-)
ReplyDeleteHow about this: The two halves of the bright and dark triangles are inversely related. The top half of the dark one is the shadow cast by the bridge; its lower half is the shadow's watery reflection. As to the bright triangle, while the sun shines directly on both its halves, the water below reflects its light to the bricks above, which are already bathed in direct sunlight.
ReplyDelete