Can you really see light diffraction when looking through your fingers at a light?

IrshadAlam

But looking through fingers works.
I mean I can very well read textual material kept just 1 cm away from my eyes when I look through hole created by joining my first three fingers. This is something which is to be taken notice of !
You may ask where did I place my fingers when the text was just 1 cm away from my eyes?
But I am a thin person and I manage anyhow to place my fingers in between eyes and text.
Believe me.
That is not just due to lenses in my eyes but it is something due to properties of light.
And to find out what that is I had left my schools after 10 th standard at the age of just 15.
But here in India I didn't get infrastructure to do so.
But today I am working on it and got some results which I have published on my website.
I believe we all can focus on some new property of light.

randmor

I noticed as a kid (with my rather poor vision), that I could bring into focus distant objects simply by forming a small hole using the tips of three "fingers". In my case, I use the thumb and forefinger of my left hand and the thumb of my right hand. I may need to move the fingers relative to each other a bit to bring the object into better focus. When doing this, the hole formed by my finger tips are a few centimeters from my eye. So, I guess this phenomenon is light diffraction. Now in my old age, I find this technique useful in seeing LED displays across the room like digital clocks and the temperature setting of the air conditioner. I also use this technique to see the number of approaching buses (those that are LED or lighted work best). This draws odd glances at times, and makes me wonder if other people know about this technique / phenomenon. It seems that some of you may have notice this phenomenon. And yes, it works well for viewing text on TV screens across the room. So, I assume we are talking the same thing.

-Rand.

Jeremy87

I've noticed this too (I need glasses), but never had any real use for it.
However, I think that's just limiting the amount of light coming in from larger angles from the object, which makes lense focus errors smaller.

The easiest way for me to notice this diffraction pattern is to be in a dark room, have the door *just slightly* open (easily adjusted to get a very long and thin opening) into a very bright room, and defocus as much as I can.

CAF123

I was studying how the interference pattern on a screen was affected by a change in refractive index of air in a Michelson set up. ( I.e how fringe displacement is affected by changes in pressure in a gas cell mounted on an interferometer.)

To observe this pattern, it sufficed for me to simply look at the screen (with a darkened room ), however to make the effect more discernible I used a lens in front of the detector. I think at some stage I had considered using a photomultiplier as well.

jbriggs444

I believe that this has nothing to do with diffraction is more along the lines of a "pinhole camera". By limiting the aperture through which you are viewing an object, you are limiting the portion of the cornea that is focusing the light from any given point on the object, thus minimizing the effects of stigmatism and both far and near-sightedness.

For a suitably small aperture you would not even need a cornea at all.

DrZoidberg

If you close one eye and hold a finger in front of the open eye while looking at the screen, you can see that there is a region around your finger in which light is being defracted. Move your finger in front of some text and you see the letters being stretched withing that region. If you look very closely you can see that there are actually several such regions, like layers of an onion, around your finger. When you bring two fingers close together you get dark lines where these layers are overlapping.

Jeremy87

I think this stretching is a result of the aperture getting smaller from only one direction.
The blur from defocusing comes from light from the source going through different parts of the aperture and landing on different parts of the retina. Eliminate one side of these light beams and you eliminate one side of the blur, concentrating a sharper image to the other side.

Emilyjoint

'You cannot see a diffraction with your eye. You have a variable lens stuck in your eye. If you remove that and let the light right onto your cornea, then yes, you can.


I do not understand this statement. I have seen lots of diffraction patterns.... with MY eyes...
Moire fringes seen with bright light through an umbrella are a diffraxction pattern ...or so i HAVE been lead to believe.
What else will you see a diffraction pattern with if not your eyes !!!!!!

ZapperZ

No, you see diffraction pattern on a screen or on a place that images that diffraction pattern. You do not see it when it passes through your eye lens directly to be imaged onto your retina. That is what is meant by that statement.

Zz.

Emilyjoint

I still don't understand what you mean.
I have looked at a lamp directly through a diffraction grating and seen the diffraction pattern. With a fluorescent light it is possible to see the spectrum formed by diffraction.
I have also seen the interference pattern by looking through double slits .
I have shone a laser pen through a diff grating and got the diff pattern on a screen, wherever the screen is placed. The spots on the screen get further apart when the screen is moved away.
I understand that everything Seen is an image formed on the retina of my eye by the eye lens.
I don't get your point about the eye !

DrDu

As far as I know, the effect is due to the laminar structure of the cornea and lens which produces several images of the same object. I don't have a reference, though.

sophiecentaur

To be fair, everything you 'see' is a diffraction pattern. The coherent pattern that forms on your retina is the sum of waves which have taken many different paths from each particular small region on an object and add up, mostly, in phase, in corresponding parts of the 'image'. This is what happens with a properly 'designed' optical system with suitably wide apertures involved. It just so happens that some diffraction patterns, under special conditions of small apertures or repeated patterns, exhibit fringes, rings and dots. These are selectively, but a bit sloppily, referred to as diffraction patterns.
What you see between your fingers consists of vertical strips and you can see the same effect when you look through a small slot in a thin sheet - so it is hardly likely to be due to multiple reflections, as suggested.
Are people doubting the 'diffraction' explanation just because light has such a short wavelength that it 'just can't be true'? There are plenty of diffraction / interference effects in everyday life. One very dramatic effect is the array of dots you see through the fabric of an umbrella (suitable type of yarn, I expect) when you look at a distant street lamp. You're out one rainy night, full of beer and good will and, before your eyes, is a bit of basic Physics that no one ever remarks on.

CWatters

Not diffraction. I believe this is similar to the effect people are seeing (only with two eyes)..

http://www.sandlotscience.com/EyeonI...nalFoolery.htm

DrDu

Why doesn't anybody of you guys take out his cool iPhone and takes a photo of these "interference" fringes?

DrDu

I think a similar phenomenon is encountered in microphotography, the so called "Coolpix ring artefacts". See here for an explanation:
http://www.klaus-henkel.de/coolpix-artefact-report.pdf

mfb

This is true if you focus your eyes on the fingers, but if you focus it on the light source behind them you do not get an image of the slit.


Just to check some numbers:

10cm distance eye<->fingers, 200µm slit width (+- factor 2), ~80cm distance from an LCD monitor, light of ~500nm wavelength, ~3mm width of the eye, ~3cm visible area on the screen.

3mm/10cm corresponds to the maximal angle between two light rays hitting the eye, this is close to the ratio 3cm/80cm. As further confirmation, the visible area does not depend significantly on the slit width, it just changes the brightness. Moving the fingers closer to the eye increases the visible area.

Using those values, single slit interference pattern have a separation of ~0.25mm at the lense. However, even the first side-maximum has an intensity much smaller than the main maximum (see here, for example). This could be somehow compensated by the nonlinear response of the eye to light, but with a bright light source (like a white area on an LCD screen), we would get the superposition of many single-slit patterns. In addition, different wave lengths would diffract in different angles, so you should see colors by looking at an LCD screen.



In the setup, the eye is adjusted to produce constructive interference for a light source 80cm away. First the regular vision, then the vision through the slit only:





As you can see, the condition for positive interference remains the same, with and without slit. No new diffraction happening anywhere. But we get a reversed image of the lense and other parts of the eye. This lead me to another test, move the fingers closer to the eye to see it clearer: Keep the separation of the fingers constant, move them in your field of vision. You will see that the lines "move" quicker than your fingers, in confirmation with the inversion in the eye. At the same time, this rules out any diffraction/interference effects from the slit between the fingers, as the position of lines would be fixed there.

I do not know what exactly produces lines. But it is certainly a part of the eye. Apparently absorption/scattering in the eye is not the same everywhere.

sophiecentaur

You called my bluff so I had to do something about it!

Naturally, I couldn't get an effect when I simply tried photographing the slit. The slit image came out fringe-free and with variable degrees of fruzziness, depending upon the camera focus. I did this with a Pentax DSLR with a range of lenses and got the same result. I decided it must be something to do with the aperture. Even with an f32 qperture setting, the image looked ok. But, of course, the light path through a camera lens is complex and I am not sure how the iris actually works. So I improved the experiment with a bit of effort - but not too much (I am not a total nerd).
First, I cut a neat(ish) slice in some thin black card for the slot and made a 'pinhole' in another piece of black card.
The pinhole was taped across the front of the lens and I held the slot at various distances and used various focus settings. The best result I got (it needed to be against a white wall in bright sunshine) is attached. It shows the same sort of pattern that I see between my fingers. I guess I could improve on it with a cleaner pinhole aperture, a more appropriate slot width and, possibly a different lens.
I think the picture is good enough to show that the phenomenon is not just 'an eye thing'.