Lines Appear When Staring at Bright Lights - What's the Explanation?

[bigjohn45]

My physics master at school (1970s) said that if you hold your hand in front of your eye whilst looking at a bright light, you can see lines appear in the tiny gaps between the fingers. He said that though some people thought this was slit diffraction, they were mistaken, the effect actually being caused by something taking place in the eye. Thirty years later I find physics teachers are telling kids that this effect is slit diffraction. Who is right, and what, if any, is the real explanation of these readily observable lines?

 

[Dadface]

Interesting question and my first impressions are that your teacher was correct.If it was slit diffraction you would expect to see a bright central maximum surrounded by subsidiary maxima and minima which is not what I observed.Also you would expect to see the pattern spread out when the slit width is reduced and I did not observe that either.I am not ruling out diffraction but at the moment I feel this is very unlikely.As to what the real explanation is ,well that requires a bit more thought.

 

[Andy Resnick]

I don't think (noticible) diffractive effects will occur, although it would be easy to check- instead of looking at a bright light, put your fingers in front of a laser (or other monochromatic source) and project against a wall. Look for fringes- which will not be very stable since we can't hold our hands stable enough.

My suspicion is that the optical effect is caused by blurring- my fingers are too close to my eye to focus on.

 

[rcgldr]

Light will diffract around any relatively sharp edge, but it's not that noticeable with white light unless the edge is very sharp and the distortion layer is thin. However in monochromatic light, moderately sharp edges will cause significant diffraction, including small holes (much larger than pin holes):

diffraction.pdf

I seem to recall that if you use the stick from a soap bubble blower and look at black lighted source, the distortion through the open hole is significant compared to white light.

I'm not sure what's going on with softer edges like your finger tips.

 

[mgb_phys]

Simple test - if it is dffration you can take a photo of it.
If it's an effect of you fingers trembling and your eye tracking them and the rates beating to block and unblock the source then you won't see anything in the photo

 

[ZapperZ]

My physics master at school (1970s) said that if you hold your hand in front of your eye whilst looking at a bright light, you can see lines appear in the tiny gaps between the fingers. He said that though some people thought this was slit diffraction, they were mistaken, the effect actually being caused by something taking place in the eye. Thirty years later I find physics teachers are telling kids that this effect is slit diffraction. Who is right, and what, if any, is the real explanation of these readily observable lines?

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.

Zz.

 

[mgb_phys]

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.

However you can see a laser speckle pattern with your eyes

 

[cesiumfrog]

You cannot see a diffraction with your eye. You have a variable lens stuck in your eye.

What? Why do you think a variable lens should make it impossible to see a diffraction pattern? How do you reconcile this with the our use of spectroscopes? And what is your alternate explanation for the phenomena in question? (Unjustified absolutist positions produced the DDWFTTW fiasco..)

At first look, the fact that the number of stable lines between my fingers depends strongly on the depth that I focus my eye would seem like evidence in favour of the diffraction interpretation.

Oh, and cornea? Did you mean retina?

 

[bigjohn45]

OK, so nobody can give me a clear answer. Here's a hypothesis: it's caused by reflections. What I see myself is a number of duplicate images of the edge of my finger. Could each part of the skin, in between the cracks of the fingerprints, be flat enough to cause an individual reflection? The angle of incidence is almost parallel to the skin, so light should in theory reflect well. But I can't think why it only works when the fingers are out of focus.

 

[VelociBlade]

I don't think (noticible) diffractive effects will occur, although it would be easy to check- instead of looking at a bright light, put your fingers in front of a laser (or other monochromatic source) and project against a wall. Look for fringes- which will not be very stable since we can't hold our hands stable enough.

My suspicion is that the optical effect is caused by blurring- my fingers are too close to my eye to focus on.

I have to agree with this guy - I thinks it has more to do with a person's sight ability than it has to do with diffraction.

 

[physics girl phd]

I agree too. Often when objects are up close you'll see double, since each eye sees a different angle but the large difference in angle makes it more difficult for the brain to process correctly (you really have two eyes for good depth perception... because a small difference in angle your brain can sort out).

 

[ZapperZ]

What? Why do you think a variable lens should make it impossible to see a diffraction pattern? How do you reconcile this with the our use of spectroscopes? And what is your alternate explanation for the phenomena in question? (Unjustified absolutist positions produced the DDWFTTW fiasco..)

At first look, the fact that the number of stable lines between my fingers depends strongly on the depth that I focus my eye would seem like evidence in favour of the diffraction interpretation.

Oh, and cornea? Did you mean retina?

Yes, I did mean the retina.

If you put a lens after a double slit, you get back the image of the double slit at the focal point. What the lens does is do an "inverse Fourier transform" of the diffraction pattern (the diffraction pattern is really a Fourier transform of the slit).

A "variable lens" is simply a means to be able to focus onto the retina. If you try to look at the light coming from a double slit with your eye directly, you'll see the double slit. You don't see interference pattern.

Zz.

 

[cesiumfrog]

pattern between my fingers depends strongly on the depth that I focus my eye

A "variable lens" is simply a means to be able to focus onto the retina. If you try to look at the light coming from a double slit with your eye directly, you'll see the double slit. You don't see interference pattern.

If you had taken a moment to close one eye, hold up your fingers, and focus at different distances, then you would have see that the pattern disappears if you bring the aperture into focus. This nullifies your argument.

 

[ZapperZ]

If you had taken a moment to close one eye, hold up your fingers, and focus at different distances, then you would have see that the pattern disappears if you bring the aperture into focus. This nullifies your argument.

Have you tried putting a lens at the focal plane after a slit? What image do you see? The diffraction pattern?

Zz.

 

[cesiumfrog]

According to http://planetquest.jpl.nasa.gov/technology/diffraction.cfm" , it is indeed a diffraction pattern:

All you need is a source of light, such as a fluorescent or incandescent light bulb. Hold two fingers about 10 cm in front of one eye and make the space between your fingers very small, about 1 mm. Now look through the space between your fingers at the light source. With a little adjustment of the spacing, you will see a series of dark and light lines. These are caused by constructive and destructive interference of light diffracting around your fingers.

Zz's objection (that a diffraction pattern does not form behind a lens) http://books.google.com.au/books?id=HtBv6pvHFNMC&pg=PA154". (And, bigjohn45, it can't be fingerprints because it works between cards of paper too.)

So far all evidence seems consistent with a Fresnel diffraction pattern. Can anyone suggest any other explanation specifically?

 

[Dadface]

Strictly speaking interference and diffraction are two different aspects of the same thing the minima of a diffraction pattern being at locations of destructive interference etc.The pattern I see bears a closer resemblance to a 2 slit interference pattern or the pattern from a grating than it does a single slit diffraction pattern, but there are puzzling features:
1.There seems to be no intensity variation across the maxima .
2.There seems to be no change of fringe separation when the gap between the fingers is changed.
I think bigjohn 45 made a relevant point in post number 9 and I think reflections may play a part in this.I am not at all convinced by NASAs explanation and I feel there is more to this than meets the eye.
bigjohn you have a lot to answer for -last night when I should have been relaxing with my martini my wife caught me acting like a total lemon looking through my fingers at the TV.

 

[ZapperZ]

bigjohn you have a lot to answer for -last night when I should have been relaxing with my martini my wife caught me acting like a total lemon looking through my fingers at the TV.

You had must made me spit my coffee all over the keyboard! You should be punished for this! :)

Zz.

 

[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 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.

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 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.
-Rand.

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]

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.

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]

'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 !

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/EyeonIllusions/PersonalFoolery.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]

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.

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]

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

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'.

 

[sophiecentaur]

Woops - here is the atttachment.