Light—and all waves—can bend around the corners of obstacles found along its path. Because of this phenomenon, called diffraction, it is impossible to focus light onto a spot that is smaller than half its wavelength. In other words, the highest resolution one can theoretically achieve using an optical microscope is approximately 250nm, a barrier called the diffraction limit. Unfortunately, this resolution is not enough for observing fine cellular structures, such as those found in neurons.
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Source: Phys.org