The resolution of a microscope is limited by the diffraction of light. Diffraction is a phenomenon where light waves spread out and interfere with each other when passing through small openings or encountering obstacles. This spreading out of light limits the ability of a microscope to distinguish between closely spaced objects.
The resolution of a microscope is determined by a property called the Rayleigh criterion, which states that two closely spaced objects can be resolved if the central maximum of the diffraction pattern of one object coincides with the first minimum of the diffraction pattern of the other object.
In the case of a regular microscope using visible light, the wavelength of visible light ranges from approximately 400 to 700 nanometers (nm). Objects smaller than the wavelength of visible light are typically on the order of nanometers or smaller. When such small objects are illuminated by visible light, the diffraction patterns generated by their interaction with light overlap significantly. As a result, the central maximum of one object's diffraction pattern will not coincide with the first minimum of the other object's diffraction pattern, and the objects cannot be resolved as separate entities. This phenomenon is known as the diffraction limit.
In addition to diffraction, there are other factors that can limit the visibility of small objects using a regular microscope, such as the numerical aperture of the microscope's lenses and the presence of aberrations. However, diffraction plays a fundamental role in setting the limit on the resolution of a microscope.
To visualize objects smaller than the wavelength of visible light, specialized microscopy techniques such as electron microscopy, scanning probe microscopy, or super-resolution microscopy are used. These techniques employ different principles and imaging modalities that overcome the diffraction limit, allowing for higher-resolution imaging of nanoscale objects.