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Why can't we see objects smaller than wavelength of visible light?

  1. Mar 22, 2013 #1
    I read in Feynman's book that it is not possible to see objects less that wavelength of visible light with a microscope. That's the reason why we go for electron microscope. Why can't we objects which is less than wavelength of the information carrying medium (which in this case is visible light)?
     
  2. jcsd
  3. Mar 22, 2013 #2

    jedishrfu

    Staff: Mentor

    objects smaller than the highest frequency of visible light can't be seen because they wont reflect back any photons for your lights to respond to. If its visible in an ultraviolet frequency then our eyes wont detect it.

    But researcher have done some work in this area to improve resolution:

    http://news.sciencemag.org/sciencenow/2011/04/scattering-microscope-peers-into.html [Broken]
     
    Last edited by a moderator: May 6, 2017
  4. Mar 22, 2013 #3
    Thanks. But once again one doubt. Why don't they reflect back the photons? Do they get scattered?
     
    Last edited by a moderator: May 6, 2017
  5. Mar 22, 2013 #4

    jedishrfu

    Staff: Mentor

    its like the object isn't there the wave just goes through it until reflected by something matching the wave length.
     
  6. Mar 22, 2013 #5

    Cthugha

    User Avatar
    Science Advisor

    What you "see" is usually scattered light, not reflected light. Reflection is not even a well defined concept for very small particles like atoms or molecules.

    You may be able to see the effects of light scattering off such small particles (like the sky being blue), but you will not be able to resolve these particles using light of large wavelength. Whether light gets scattered or not depends strongly on the size of the scatterer. In the limit of tiny particles, the scattering process is called Rayleigh scattering. In the simple case of a round particle of size and light of wavelength w, the scattering probability behaves proportional to d^6/w^4. So the scattered intensity becomes small quickly as the particle size becomes small compared to the wavelength.

    Nevertheless, there still is some scattered light, but that does not allow you to resolve the scatterer. The optical resolution of a microscope depends on the wavelength w used and the numerical aperture (NA) available. The resolution is around 0.61 w/NA. Using standard visible light microscopes, this gives you a resolution of roughly 200 nm in the best case.

    The underlying concept limiting your optical resolution is in this case diffraction. If you look up the diffraction pattern of a circular aperture, you will find that it will be a so-called Airy disk. So the light coming from the aperture will not be imaged to a spot, but to such an Airy disk with the central spot having a certain diameter depending on the wavelength used and the size of the aperture. If you use two apertures, you get two such disks. If their central spots overlap strongly, you cannot distinguish whether you had one or two apertures to start from and your resolution is insufficient to resolve the object.
     
  7. Mar 22, 2013 #6
    Diffraction is the basis of the explanation. Everything you see is basically a diffraction image. When the object is close to the size of the wavelength it is not easy to identify a 'physicsl' shape.
     
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