An old, oft-repeated truism: The size of the finest detail that can be resolved is proportional to L/NA, where L is the wavelength of the light and NA is the numerical aperature (related inversely to f number (f/#)).
BUT: Confocal, I5M, and the new 4Pi can go far beyond this. 4Pi resolves detail below 100nm using a system of genius parallel high NA (~1.4) optics and deconvolution.
left-handed material, metamaterial
negative refractive index
optical diffraction limit
Schlieren, shadowgraph and interferometric techniques have been used for many years to study the distribution of density gradients within a transparent medium.