After months spent in a quiet corner, with nothing but my blasting headphones on, yesterday morning I returned to work. Afresh, I started with a number of quick visits to some of my usual online friends and foes, when suddenly I found myself in the midst of my first ever Groundhog Day. The place was this. The time was, almost to the day, ten years later. And, sure enough, the subject was the same, the cast, some props...😮😇😈... Look, let me tell you this right from the outset: Read first the story in the link I gave you to see if it really is your own kind of tea, or not. Then, if it is, I urge you to read next its decade older counterpart, for I truly believe that it is worthy of attention, and deliberation. That's all I'll ask of you, before advancing any further than the mark below. Unless, of course, you happen to be Secular Sanity (aka Debstar). Of you I will ask nothing, please proceed.
The first thing that caught my attention about this thread was the manner in which the OP (who is someone I know quite well, as a matter of speaking) began the proceedings. To my mind it was highly unusual for SS to set off a discussion, on a subject I know she cares a great deal about, with such a hurriedly spat out introduction:
When you look at the following image through a prism with the apex (thinner portion) pointed towards the left or the right, why do you see magenta on one side and green and black on the other?
Keep in mind that the spectrum is reversed.
Let us jump from here to post #5, which is the first reply to SS' question by Ibix, who is credited with being a Science Advisor and an Insights author at the Physics Forum. Ibix begins his reply with the following remark:
Well, refraction will cause the red and blue parts of the image to overlap in places. The spectrum of light your eye receives from the overlap region is the same in the two cases, so there's no physical reason why you'd see different colours
Unsurprisingly (in as far as I'm concerned) this science advisor has not a clue about the issues at stake, yet like many others before him he did not hesitate to come up with a definitive statement, without even bothering to contemplate the possibility that the reigning theoretical understanding might just happen to be deficient in dealing the matter at hand. And if the cited paragraph is not enough to convince everyone than they should pay close attention to the second part of Ibix's reply, in which he mentions a certain dress that had divided the world a few years ago into two halves--one brown, the other one blue. Anyway, regardless of what your opinion or mine may be about Ibix's reply, the reality is that one particular participant in that discussion, by the nickname of sophiecentaur, raised his thumb in approval at the end of the post, before laying down his own offer upon the common table.
I'll overlook sophiecentaur's post, for it is totally inconsequential to our story. Nonetheless, it was so refreshing to see that at its end there was Ibix's thumb now raised, in a good turn of reciprocal acknowledgement. Nice.
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Secular Sanity 's post #7:
Yeah, I thought about the blue dress but that revealed differences in human color perception. The camera picks up the magenta, the green and the black, as well. Blue and red create magenta but why are we seeing green on the opposite side when I have the green set at zero, and, in particular, why are we seeing the black line?
(At this point SS posted one half of the image you saw at the start of the thread, namely the one shown below on the left, and to that I then added my rendition of what an observer will see through his prism, oriented with the apex pointing to the left, from a distance of about 0.5 m.)
Not that it would have mattered much, anyway, for it was as clear as daylight that they had no idea about how that spectral display came to be. But the most interesting part of all that saga was that the only person who had some knowledge about that had obviously opted to make no mention at all in that regard either. (Not directly, or candidly, or specifically, in any event.)
Taking into consideration all these factors at this point I will stop addressing each and every little 'pearl' that everyone had contributed to the lot, choosing instead to merely mention some of the bigger and more relevant ones as we'll begin dissecting each and every significant aspect of the real factors and facts that are working in earnest well behind the scene.
Some regions make sense. For instance the left hand of the blue rectangle appears, reasonably, to be shifted to the left.
Why is that, sophiecentaur? Why is it reasonable that the blue rectangle appears to have been shifted to the left? Do you know why? Because if you don't, I'll tell you. The answer to that question is because in subjective experiments a prism displaces blue objects--when they are cast against a black (dark) background--in the direction of its apex. This is the first major factor that is working behind the scenes in all subjective prismatic experiments.
Why are there so few straight lines?
Really Ibix? Do you really need to ask that question? Have you never looked at the world through a prism?
My first comment is to wonder whether the Blue is actually (0R, 0G and 255B). If it isn't then there can be some R and G in it which are shifted less. I just looked with my 'dropper' tool and it shows that the Blue colour actually has about 10% additional (pollution) R and G. So the blue is far from saturated. The Red, likewise, is not pure but has 10% additional B. Using saturated colours would have been better and avoided us chasing possible red herrings.
If you are trying to find some reason for that irksome G that you've been all complaining about, I can assure you that its seemingly annoying appearance has nothing to do with any apparent pollution or deficient saturation of B. In fact, the reality is that the reason for its appearance is classically and wholesomely Newtonian. Which means that anyone in your position should have figured that out a long time ago already. I will tell you all about that in a moment, but first I'd like to let you know what the third major factor in subjective prismatic observations is.
The third major factor at work in subjective observations is that G is not displaced at all by the prism, and as such it complements in a beautiful fashion the other two, we have mentioned.
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Now, about that 'troublesome' dark green, which apparently baffled everyone and which, for all intents and purposes, appears to have remained a mystery to the end of the thread. Have a look at the image above on the left, paying attention only to the upper red rectangle, which is laying with one half on a white background and the other on the blue. The upper half of that rectangle is bordered on the right by a band of magenta and on the left by a dark red strip and by a wider, yellow one. Let me now ask you this question: Are you at all surprised by the apparition of that red-yellow combination of colours? If your answer is yes, then you are truly in the wrong place. If your answer is no, have a good look at that dark green patch right below the yellow stripe and tell us if you (still) find its being there strange, mystifying, or surprising at all. If your answer is no, I'm glad you saw how simple, clear and obvious is the reason for its appearing there. If on the other hand your answer is yes, I'll ask you to have a close look at the picture below.
