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There's several ussues with this argument:

- If the interference pattern was explained by diffraction by a semi-infinite plane, why don't I see it when using only one finger? I only see a blurry shadow. The second finger is needed to make the pattern appear.

- All formulas that are used compute the light intensity projected on a screen. In the actual experiment, we're looking at the slit through a lens (our eye or a camera). That's not the same thing.

- The fact that this is white light interference is handwaved away. To model it correctly, you'd need to compute what happens at each wavelength, then integrate the resulting spectrum multiplied by CIE's x, y z functions at each point, and finally do a bunch of math to bring that in the sRGB color space if you want to display the model's result on a screen.

[Too much math before my first coffee in the morning.

Anyway, the harder I try to write a rebuttal, the harder it gets. Now, assuming a .1mm gap (1/32 inches) and 500nm visible light, the article translates to something like ~A 200 wavelengths single slit can be approximated as two semi-infinite screens~ that makes a lot of sense.

But I may be missing something.]

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Back to your questions:

> - If the interference pattern was explained by diffraction by a semi-infinite plane, why don't I see it when using only one finger? I only see a blurry shadow. The second finger is needed to make the pattern appear.

I got the best results with a led lamp on the wall of my kitchen. It's a 5x5 inches square, with a white translucent plastic and a metallic frame that hides a led strip and can be bought in any electricity store near my home. I was standing like 10 foots away, in a slightly dark area (or use the other hand to cover the eyes and fingers).

I almost put my finger like 3 inches away from my eyes, I close the fingers until I see the pattern. Then I open them slightly and I see a phantom line around my finger that does not disappear when they are far away from each other. If I look very carefully, I see a second phantom line.

- All formulas that are used compute the light intensity projected on a screen. In the actual experiment, we're looking at the slit through a lens (our eye or a camera). That's not the same thing.

A standard trick is to use a lens and a screen at the focus distance instead of a screen at infinity. This is similar to the eye. The lens in the eye will have a different adjustment to make a clear image of the eye, so it will not be exactly equivalent. But it's close enough. I'd not worry too much about this part.

> - The fact that this is white light interference is handwaved away. To model it correctly, you'd need to compute what happens at each wavelength, then integrate the resulting spectrum multiplied by CIE's x, y z functions at each point, and finally do a bunch of math to bring that in the sRGB color space if you want to display the model's result on a screen.

To get a nice rainbow interference pattern, you probably need a almost puntual source of light. A diffuse source will make a blurred rainbow that is impossible to notice. But I need a diffuse source like the lamp in my kitchen to notice the dark lines because they are too weak.

White leds use phosphorus to get the full spectrum, I'd try with a lamp with leds of color that are almost monochromatic. I'd try with red and blue to maximize the distance of the interference lines, but I'm not sure if the blue leds use phosphorus too. Perhaps red and green is better for this. (Perhaps a computer screen with Painbrush filled with #FF00FF is a good alternative.)

Blue LED don't use phosphorous, their spectrum is a few nm wide, just like other colors. I wouldn't bet on the spectral quality of a random screen. OLED might be okay, but other technologies use filters in front of a white light, the spectral width will probably be wider.

-I do see it on the edge of a single finger.

-I agree, also I can only observe the effect when focusing on the gap

-sure, but weirdly the effect has to be wavelength dependent, but there are no color fringes.

I think this is something else, but haven't figured it out yet.

Interesting, I can only see the bands when holding my fingers very close to my eye, and _not_ focussing on it. If I hold my fingers far enough to be able to focus, I don't see them. Maybe my eyesight is not good enough. Focussing on a single finger, I see that the border has a green tint to it.

I agree that there's no colour in the fringes, which is unexpected for white light interference.

>-sure, but weirdly the effect has to be wavelength dependent, but there are no color fringes.

I do think you can get colour fringes in some circumstances. Try doing it in a dark room with a bright light coming through a small gap (e.g. between curtains). Like:

                                |
                                |                    (dark room)
                                |

    light source - - - - - ->- - - - - - - - - ->- - - - - - ->- - - - - - - - - ->  eye
                                                                       |
                                |                                      |
                                |                                      |
                                |                                    finger
                        small-ish gap (1-5cm)

IIRC you can get colour fringes between the finger and the top edge of the gap behind it.

EDIT: I just tested it, there is definitely a rainbow spectrum between the finger and the gap. The gap side is blue and the finger side is red. Not sure if this is the same effect as the article though.

One should be very careful when discussing diffraction to make sure you’re about what you think you’re talking about. Case in point: Figure 5. If you look at the drawing, and you contemplate the line segments you’ve drawn, you may find that it’s quite hard to justify the supposed path difference of lambda/2. In fact, the path difference is zero. You need a different drawing for lambda/2, and you might need to relate your drawing to an actual physical scenario (projecting on a wall, for example) to see which approximations are valid.

As for the finger slit, I’m quite suspicious that the actual visible effect isn’t diffraction from the fingers at all — my intuition is that it’s more likely to be diffraction from your eye or perhaps something more complex in the combined system. You would need to extend the drawing to include a light source (perhaps at infinity), the finger slit, and some credible model of an eye including, at least, a finite pupil, a lens, and an image surface (the retina) that may or may not be quite lined up with the focal plane.

Another fun effect: on a sunny day or under a ceiling light, if you close your eyes part way and hold your head at an angle such that you can’t quite see the sun, you can often see fun color bands where the colors change from left to right. This is diffraction from your eyelashes :)

Slightly related, if you move such a slit or similarly a pinhole at the same distance, you can see your own retinal vasculature. It's only visible if the hole moves because it presumably triggers the motion contrast neurons.

Thank you so much for posting this!

I've always been so fascinated with this phenomenon since I was a kid and would spend a long time looking at this lying on my back in the sun.

I remember telling an adult sometime and they said very authoritatively that light wouldn't bend noticeably at that scale and that it was probably some kind of optical illusion, and accepting that at face value.

But now I'm looking at it again and it's so fascinating and there's so much there – love this conversation! Especially pleased that there's no scientific consensus about this.

!!!

I've been bothered by this for decades. I remember arguing with my HS physics teacher that this couldn't possibly be single-slit diffraction, for the same reasons the article brings up. I was never able to figure out a satisfactory answer for what it really was, even after a physics degree. Feels good to be vindicated :D

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I don't think telling people to directly look into sunlight is good advice