No, atoms cannot be directly observed using conventional optical microscopes because their sizes are much smaller than the wavelength of visible light. The resolution limit of an optical microscope is dictated by the diffraction of light, which restricts the level of detail that can be observed. Since atoms are on the scale of angstroms (10^-10 meters), which is far smaller than the wavelength of visible light (around 500-700 nanometers), they are beyond the resolution capabilities of optical microscopes.
However, there are advanced techniques that enable the indirect visualization and manipulation of individual atoms. Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) are two such techniques that have been used to image and manipulate atoms and molecules at the nanoscale.
In STM, a sharp metal tip is brought very close to the surface of a conducting material. By measuring the flow of electrons between the tip and the surface, a topographic image can be generated with atomic resolution. This technique allows scientists to "see" individual atoms on conducting surfaces.
AFM, on the other hand, uses a tiny cantilever with a sharp tip to scan the surface of a material. The interaction between the tip and the surface produces subtle forces that are detected by measuring the deflection of the cantilever. AFM can provide atomic-scale resolution for a wide range of materials, including non-conductive surfaces.
While these techniques have enabled remarkable progress in the visualization and manipulation of atoms and molecules, it's important to note that they are not direct optical observations. They rely on physical interactions or effects, such as electron flow or atomic forces, to generate images and information about atomic positions and surface structures.