I have just started reading about a classical electromagnetic treatment of light-matter interaction (beginning with dispersion relations, and then moving on to the standard phenomena - reflection, refraction, etc.). The discussion begins with a forewarning that light is not 'continuous' as the classical electromagnetic treatment suggests. It touches upon the photonic nature of light, and this has got me confused: We say that light exhibits its particle nature only when it interacts with matter. But going by what quantum mechanics teaches us, isn't the matter light would interact with a wave (-particle) too? I mean, consider a grain of sand. Apparently, we cannot observe its wave-particle duality because even while sitting still, it always has a certain instantaneous momentum due to thermal vibrations, which give it a corresponding wavelength too less to be observable. Now, I want to apply 'light-logic' to this grain of sand: when light is not being observed (for example, as it propagates through vacuum) it behaves like a wave (a probability wave) and then collapses into a specific state when observed (i.e., when it interacts with 'matter'). By a similar token, one would suppose a sand particle to be a wave when not interacting with other matter or light, but it is clearly always a particle no matter what...how? I thought maybe it is because it is always interacting with 'ambient' photons, so it is always in a state of 'observation'. But then, the same should have been true for a photon as it moves between the slitted and solid planes in Young's double slit experiment - after all, it is always interacting with the particles in the air isn't it? Does being so small somehow give the photon a special status? Basically, I'm trying to analogically apply the concepts we use with microscopic particles to macroscopic objects, but things don't seem to fit. How am I supposed to imagine the wave (probability wave) of, say, a sand particle (which has vanishing wavelength)?