<P> In the field of optics, transparency (also called pellucidity or diaphaneity) is the physical property of allowing light to pass through the material without being scattered . On a macroscopic scale (one where the dimensions investigated are much, much larger than the wavelength of the photons in question), the photons can be said to follow Snell's Law . Translucency (also called translucence or translucidity) is a superset of transparency: it allows light to pass through, but does not necessarily (again, on the macroscopic scale) follow Snell's law; the photons can be scattered at either of the two interfaces where there is a change in index of refraction, or internally . In other words, a translucent medium allows the transport of light while a transparent medium not only allows the transport of light but allows for image formation . The opposite property of translucency is opacity . Transparent materials appear clear, with the overall appearance of one color, or any combination leading up to a brilliant spectrum of every color . </P> <P> When light encounters a material, it can interact with it in several different ways . These interactions depend on the wavelength of the light and the nature of the material . Photons interact with an object by some combination of reflection, absorption and transmission . Some materials, such as plate glass and clean water, transmit much of the light that falls on them and reflect little of it; such materials are called optically transparent . Many liquids and aqueous solutions are highly transparent . Absence of structural defects (voids, cracks, etc .) and molecular structure of most liquids are mostly responsible for excellent optical transmission . </P> <P> Materials which do not transmit light are called opaque . Many such substances have a chemical composition which includes what are referred to as absorption centers . Many substances are selective in their absorption of white light frequencies . They absorb certain portions of the visible spectrum while reflecting others . The frequencies of the spectrum which are not absorbed are either reflected or transmitted for our physical observation . This is what gives rise to color . The attenuation of light of all frequencies and wavelengths is due to the combined mechanisms of absorption and scattering . </P>

Materials that clearly transmit visible light are said to be