Why objects appear smaller as the distance increases?

[Meruem]

Since objects look smaller and smaller as we go far from it, is it possible at some point the object won't be visible,even if there is nothing between two objects ?

 

[phinds]

Depends somewhat on what you mean by "visible". If you mean "visible to the human eye" then definitely because at some point the incidence of photons hitting the eye will be so few that no image will be formed.

If you mean "detectable" then theoretically, no, if you really could separate them with nothing at all in between, and if you were willing to wait long enough for a photon to arrive, AND if you were conducting the experiment in a universe that was not expanding (as our is).