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Why are ordinary optical microscopes limited in their ability to observe nanoparticles?

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Gold and palladium alloy nanoparticles — nanoparticle Image showing nanoparticles of an alloy of gold (yellow) and palladium (blue) on an acid-treated carbon support (gray). These particles were employed as catalysts for the formation of hydrogen peroxide from hydrogen (white) and oxygen (red).

Ordinary optical microscopes are limited in their ability to observe nanoparticles primarily due to the diffraction limit of light.1

Because nanoparticles are typically four to seven times smaller than the wavelength of visible light, they fall below the resolution threshold of conventional light microscopy. In standard optical systems, the ability to resolve fine detail is constrained by the diffraction of light waves; generally, structures spaced closer than half the wavelength of the light used—approximately 200 nanometers—cannot be distinguished as separate points.213

When the dimensions of an object approach the wavelength of light, the objective lens cannot collect all the diffracted waves necessary to reconstruct a clear image of such minute details. Consequently, while nanoparticles may be observable in liquid samples under specific conditions, they are generally invisible to the human eye and require specialized superresolution microscopy techniques or direct imaging methods like transmission electron microscopy to be accurately detected and characterized.4521