What would happen if we would take a very small sphere around 200nm diameter and try to detect it from the most efficient optical microscope? Technically, the Rayleigh diffraction limit prevents the observer from getting a faithful image of the source object. This is indeed the standard magnification limit of optical instruments.

Though, I still wonder what would we actually see if we would happen to try to get an optical image of this object?Would the object disappear??(then would it mean that it would not be interacting with the light anymore which is a bit contradictory to the usual explanation of microscopic light electrons interactions??) Would we see a distorted unresolved spread spot ??

  • $\begingroup$ One have to take in account the background or the holder for such a tiny body. If one could make the background more homegenious as the tiny body I suspect you will see some distortion. $\endgroup$ – HolgerFiedler Apr 1 '15 at 5:12

You will still "see" the object but it will appear blurred in the shape of the so-called Airy diffraction pattern with a size larger than the actual object (dependent on the numerical aperture of the objective lens and the wavelength of the light used to observe it in the microscope). For VIS light and a 100x objective lens with an N.A. of 1.4 this is approx. 250 nm (xy plane, worse along the propagation direction of the light, z). I routinely image 80 nm gold spheres in a microscope. The problem that arises with smaller objects is that the contrast of the object with respect to the background gets in the way of still being able to see the object. Different illumination options in microscopes try to resolve this. One more thing: If you think about fluorescence microscopy, which yields a very high contrast due to specific labelling of the object of interest, you can image single molecules perfectly fine. However, the single fluorescent molecule with a size on the order of a nanometer (!) will appear as a much bigger blurred spot in the shape of the Airy disk on the imaging device/camera.

  • $\begingroup$ And, of course, getting away from a 'classical' microscope to a near field scanning optical microscope (NSOM) changes the game (and equations for resolving). $\endgroup$ – Jon Custer Apr 1 '15 at 14:43
  • $\begingroup$ when you say you image 80nm gold with 1.4 NA, what wavelength of light do you use to obtain such a resolution which is limited by $\lambda/NA$ ?? $\endgroup$ – Ronan Tarik Drevon Apr 1 '15 at 15:03
  • $\begingroup$ @RonanTarikDrevon Not entirely sure what you mean? If one uses shorter wavelength light for illumination, the appearing Airy pattern will be smaller. Consequently, following the equation for resolution in an optical instrument: $\endgroup$ – Kokomoking Apr 1 '15 at 16:26
  • $\begingroup$ @RonanTarikDrevon $$r = 0.61\lambda/N.A.$$, decreasing the NA will yield worse optical resolution/a larger Airy disk. I just use white light in a brightfield configuration so the 80nm gold beads will appear dark over a bright background. In my particular task I am not interested in achieving the best resolution when imaging those beads. Sorry for the split comment, I#m new to this. $\endgroup$ – Kokomoking Apr 1 '15 at 16:35

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