# Double-line experiment

 P: 39 If I draw two vertical lines close to each other and watch them from a short distance then I can see an interference pattern. How do you explain it ?
Emeritus
PF Gold
P: 29,238
 Quote by forcefield If I draw two vertical lines close to each other and watch them from a short distance then I can see an interference pattern. How do you explain it ?
This is vague.

Using your eyes as your primary detector is seldom a good idea. I am sure you know all about optical illusions, etc. How would you know that what you are seeing isn't an optical illusion? If it is, this is now a biology/medical science topic, not a physics topic.

Zz.
P: 3,151
 If I draw two vertical lines close to each other and watch them from a short distance then I can see an interference pattern.
I had to try it to see what you meant. I don't believe this is an interference pattern. I believe it has more to do with the eye's inability to focus on close objects. When too close the lines appear out of focus (eg spread out) on the retina. This allows the image of one line to partially overlap with the image of the other line producing a third line in the middle. This effect is similar to..

http://michaelbach.de/ot/sze_Frankfurter/index.html

P: 39
Double-line experiment

 Quote by forcefield If I draw two vertical lines close to each other and watch them from a short distance then I can see an interference pattern. How do you explain it ?
 Quote by ZapperZ Using your eyes as your primary detector is seldom a good idea. I am sure you know all about optical illusions, etc. How would you know that what you are seeing isn't an optical illusion? If it is, this is now a biology/medical science topic, not a physics topic.
Hmm, I don't know ALL about optical illusions. I think that seeing the interference pattern in this case must be an optical illusion because I just draw the lines myself.

Are you saying that this phenomenon is different from the double-slit experiment (i.e. light from two slits/lines interfering and causing the pattern ?
Mentor
P: 11,892
 Quote by forcefield Are you saying that this phenomenon is different from the double-slit experiment (i.e. light from two slits/lines interfering and causing the pattern ?
Absolutely.
P: 621
 Quote by forcefield If I draw two vertical lines close to each other and watch them from a short distance then I can see an interference pattern. How do you explain it ?
There are two critical differences here. A simple interference requires monochromatic light (as with a laser). Also, the interference pattern is projected onto a screen without any optical focusing. When view by the eye, it would be difficult to defocus your eye enough to get the interference pattern to fall on the retina.

If we set aside the double slits for a moment and simply look at a rough surface illuminated with laser light, you will see an interesting pattern of constructive and destructive interference. It's called "laser speckle".
Emeritus
PF Gold
P: 29,238
 Quote by forcefield Hmm, I don't know ALL about optical illusions. I think that seeing the interference pattern in this case must be an optical illusion because I just draw the lines myself. Are you saying that this phenomenon is different from the double-slit experiment (i.e. light from two slits/lines interfering and causing the pattern ?
Use your camera and put it at the same distance. Snap a photo. Does it see the same thing that you see?

End of story.

Zz.
 Sci Advisor Thanks PF Gold P: 12,186 There is exactly the same diffraction pattern from two lines as two slits (of the same dimensions). Both are due to obstructions in the arriving wavefront. The two patterns add up to a uniform illumination of the 'screen'. So it's not a totally crazy idea at all. The difference is in the visibility. The two slit pattern is usually viewed in subdued (or zero) ambient light and you easily see the tiny amount of energy that gets through the slits. When you are looking at the pattern from two lines, what you can see is all the incident wave, less the tiny amount that is obstructed by the lines. Visibility is many orders of magnitude less. With the right optics, it is quite possible to see the side fringes from two lines but I doubt that you were actually observing that. .Scott's comment about needing good monochromatic light is even more relevant in the case of the lines.
P: 490
 Quote by ZapperZ This is vague. Using your eyes as your primary detector is seldom a good idea. I am sure you know all about optical illusions, etc. How would you know that what you are seeing isn't an optical illusion? If it is, this is now a biology/medical science topic, not a physics topic. Zz.
Well it's the only primary detector we have got I doubt very much that the camera or any other usefull device could be built using sound smell or hearing.Maybe you could think one into existence.
Optical illusions are detected by the eye and all instruments in there manufacture are crafted from the input of vision.If you realy look at the subject closely I doubt you could without the aid of sight at some stage in your life.

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