A nanoparticle or ultrafine particle is usually defined as a particle of matter that is between 1 and 100 nanometres (nm) in diameter. The term is sometimes used for larger particles, up to 500 nm,[citation needed] or fibers and tubes that are less than 100 nm in only two directions.[3] At the lowest range, metal particles smaller than 1 nm are usually called atom clusters instead. Nanoparticles are usually distinguished from microparticles(1-1000 µm), "fine particles" (sized between 100 and 2500 nm=, and "coarse particles" (ranging from 2500 to 10,000 nm), because there smaller size drives very different physical or chemical properties, like colloidal properties and optical or electric properties. Being more subject to the brownian motion, they usually do not sediment, like colloidal particles that conversely are usually understood to range from 1 to 1000 nm. Being much smaller than the wavelengths of visible light (400-700 nm), nanoparticles cannot be seen with ordinary optical microscopes, requiring the use of electron microscopes. For the same reason, dispersions of nanoparticles in transparent media can be transparent