With some of the largest single molecules, like polymers, they can actually be seen by the naked eye or through a microscope. Most molecules, however, are so small that a single isolated molecule cannot be seen with even the best microscopes. There is a physical limitation that prevents their observation with light, which has to do with the size of small molecules (lengths of ca. 0.1 to 1.0 nm) being significantly smaller than the wavelengths of visible light (400 to 700 nm). Other techniques based on diffracting electrons off of molecules, measuring the force molecules exert against a very small metal tip, and other methods have been developed to image small molecules, but it’s impossible to see most small molecules with light in the way that we conventionally see things.