Interference Pattern Formed in a Finger Gap Is Not Single Slit Diffraction

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 :)

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.

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.

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.

!!!

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