Unusual Optical Phenomena....

ic-racer · May 24, 2020

There was a paper published in 1967 by M.G. Minnaert on unusual optical phenomena. Too bad the pictures in the reproduced article are so poor, the phenomena can't usually be seen. However, the verbal descriptions in the paper are detailed.

I have had some unusual things show up in my work and have seen some things that are hard to explain. Almost everyone has seen and photographed a mirage, but what other things have people photographed that are hard to explain.

I have an example here. Look at the EXIT sign over the door. The battery box is above the sign. However the shadow shows the battery box is below the sign.

The explanation on this one is really good because it involves two different physical optical phenomenon and two sepearate optical illusions!
The optical illusions conceal the physical optical phenomenon.

ic-racer · May 24, 2020

FYI the image was taking almost a year ago. I was actually there and went to the door to cast some shadows with my hand. It still took this long to finally get it all figured out (which will be revealed later).

reddesert · May 24, 2020

ic-racer said:
There was a paper published in 1967 by M.G. Minnaert on unusual optical phenomena. Too bad the pictures in the reproduced article are so poor, the phenomena can't usually be seen. However, the verbal descriptions in the paper are detailed.

FWIW, Marcel Minnaert wrote a classic book on natural optical illusions and atmospheric phenomena, "Light and Color in the Open Air," https://store.doverpublications.com/0486201961.html . I refer to it often. There are photos, but the reproductions aren't great, as one may expect from a reprint of a book originally published in the 1950s. Apparently it has now been updated with a large number of new and color photos: "Light and Color in the Outdoors,' https://www.springer.com/us/book/9780387979359 Either edition can be found at the usual book sources.

Richard Man · May 24, 2020

I see a light fixture that could be mistaken as a battery box, and the shadow of the sign below the exit sign?

ic-racer · May 24, 2020

Richard Man said:
I see a light fixture that could be mistaken as a battery box, and the shadow of the sign below the exit sign?

The exit sign is in two parts. A white rectangular battery box connected to the ceiling by a shaft and the actual illuminated sign with the red letters EXIT hanging down from that. The batteries allows the sign to illuminate in a power outage and the hanging shaft has electrical wires to keep the batteries charged. But none of that is in the solution to the puzzle of why the shadow shows the letters connected to the ceiling by the shaft and the battery box below. Based on the shadow on the carpet, how would the electricity get to the battery box to charge the batteries??

AgX · May 24, 2020

I hope you give us some time to sleep over that and find the answer before you unveil it...

ic-racer · May 24, 2020

I might even have a give-away for someone that gets all the details.

Just to emphasize what I mean by "optical illusion" without knowing, I made an optical illusion with the red circles above. See how they look different sizes; but they are the same size (but that particular illusion has nothing to do with the answers).

ic-racer · May 25, 2020

I will propose a question to stimulate thought.

"Does Glass Cast A Shadow?"

iakustov · May 26, 2020

Angle of refraction is different for the sign?

Nodda Duma · May 26, 2020

A nicely captured optical phenomenon arising from the construction of the exit sign itself. The view of the bricks through the exit sign gives it away for me. It indicates an optical wedge between the (not quite parallel) front and back surfaces of the exit sign, bending the path through the exit sign in a slightly different direction compared to the direct path of sunlight elsewhere. The bright “EXI” is superimposed on the battery box shadow (and upper half of the “shadow” of the sign itself) . Below the “EXI” on the floor is the “shadow” of the clear acrylic sign itself... the light through the sign has been refracted away from that area, leaving an absence of sunlight.

ic-racer · May 26, 2020

Nice job Nodda Duma!!!

ic-racer · May 26, 2020

So the shadow on the carpet is the shadow of the clear sign. Light does not pass directly through it, so it casts a shadow. This looks like it would be the battery box but it is not.
The shadow of the actual battery box is 'erased' by the projection of light through the 'prism' created by the wedge shaped acrylic sign.

Shadow of the clear sign in RED. Shadow of the opaque battery box in GREEN.

ic-racer · May 26, 2020

Two months away from the one-year mark for the phenomenon. I'll see if it gets re-created. As of now the angle of the sun is never correct to make the illusion.

I eat lunch at that table where the photograph was taken or the ones to the right, for 15 years. Have never seen the illusion before.

ic-racer · May 26, 2020

This is one of the optical illusions. It looks like the pole that supports the battery box would connect to the projected shadow (making it look like the battery box) which can add to the confusion of what is going on. The other is that the refracted projected image partially obscures the clear sign's own shadow. Thus making the shadow narrow, like the battery box.

Vaughn · May 26, 2020

I just think it is due to the difference in refracting ability of air vs plastic, and the fact that the sign and the battery box are not on the same plane.

Nodda Duma · May 26, 2020

Full disclosure: I was thinking if I got that wrong, I can’t claim to be a lens designer lol

I think I estimated the wedge has an angle of a little over 1 degree.. which translates to ~0.1”-0.12” difference in thickness top to bottom. Be interesting to know how close that is.

ic-racer · May 26, 2020

Vaughn said:
I just think it is due to the difference in refracting ability of air vs plastic, and the fact that the sign and the battery box are not on the same plane.

Though, as Nodda has figured out, just refraction, won't shift the image that much. The sign needs to be a shallow prism. I think this is pretty rare, most exit signs like that are planar.

In fact, based on Nodda's calculations, maybe that particular sign is exhibiting a slight manufacturing defect.

Unusual Optical Phenomena....

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