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I keep hearing this rule that an object must have a bigger size than the wavelength of light in order for us to see it, and though I don't have any professional relationship with physics, I want to learn the explanation for this. Also I may have expressed my question wrong, but I hope you get the idea of what I'm trying to ask.

Can you explain this as simple as possible for a non-physics person?

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Hi davsan, and welcome to Physics Stack Exchange! Actually this is an excellent question as it is. (One thing I would suggest is to put your question into the body of the post as well, so that in someone who didn't read the title would still understand it.) –  David Z Feb 8 '12 at 20:42
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@DavidZaslavsky Thanks David :] I edited the body now. –  davsan Feb 8 '12 at 20:44
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Do you want to simply detect the object, or resolve the object's structure? You can detect a point in theory, just by looking for scattering, you can't resolve two points to know if they are two points of one when they are too close. –  Ron Maimon Jul 27 '12 at 6:42
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3 Answers

up vote 4 down vote accepted

At David Zaslavsky's suggestion I'll transfer this from the comments to the answers (I was a bit hesitant because I don't know how reliable youtube videos are to still be around in, say, 6 months time!):

This little youtube video might help. You can only resolve the objects by looking at the reflected waves. The amount of detail you can get in the reflected waves can't be smaller than the wavelength (roughly speaking).

Edit: The video shows incident waves being reflected off small irregularities in the surface at the bottom of the picture. The first case (wavelength smaller than the irregularites) shows information about the irregularities being "fed back" in the reflected waves: enter image description here

The last case (wavelength larger than the irregularities) shows much coarser information being fed back, making it not possible to get any information about, for example, the size of these irregularities:enter image description here

Of course snapshots are a bit hard to read, you'd really have to look at the statistics of the received reflected waves as a function of position to really see what was going on, but the video gives a general impression of the problem.

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I think YouTube is pretty good at keeping things up except in cases of alleged copyright infringement. In any case it's not such a problem if you also include a brief explanation of what one is supposed to get from the video ;-) –  David Z Feb 9 '12 at 8:28
    
Thanks @twistor59, the answer is much more clear now with the pics. –  davsan Feb 9 '12 at 12:51
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Classically it's hard to resolve detail in an object in a less than half the wavelength of light (abbe limit).

It is possible to make an 'image' of the structure of an object if you can get closer than a wavelength from the object - near-field microscopy, essentially by measuring the electric field of the the light directly rather than focussing it.

And if you can make a material with a negative refractive index then you can image structures much smaller than a wavelength, see: The Perfect Lens and superlensimaging

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Your formulation is not accurate.

Actually an object may be seen even if it has a size of order of magnitude of the wayvelength, and even less. This all depends on what do you mean by "see it".

Simply speaking all the "wave" effects of photons have impact on the "image", which depends on one simple parameter: the ratio between the wavelength and the object size (and the distance where they're observed).

If the order of magnitude of the object size is much bigger than the wavelength - you get a "classical" image. OTOH as the object size approaches the wavelength - the wave effects start to dominate.

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By "seeing" I meant getting information about their sizes, structures, and so. –  davsan Feb 8 '12 at 21:01
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