Un material presenta transparencia cuando deja pasar fácilmente la luz. La transparencia es una propiedad óptica de la materia, que tiene diversos grados y propiedades. Un material es translúcido cuando deja pasar la luz pero no deja ver nítidamente los objetos. En cambio, es opaco cuando impide el paso de la luz. Generalmente, se dice que un material es transparente cuando es transparente a la luz visible. Para aplicaciones técnicas, se estudia la transparencia u opacidad a la radiación infrarroja, a la luz ultravioleta, a los rayos X, a los rayos gamma u otros tipos de radiación. Según la mecánica cuántica, un material será transparente a cierta longitud de onda cuando en su esquema de niveles de energía no haya ninguna diferencia de energía que corresponda con esa longitud de onda. Así, el aire y el vidrio son transparentes, porque en sus esquemas de niveles de energía (o bandas de energía, respectivamente) no cabe ninguna diferencia de energía del orden de la luz visible. Sin embargo, sí que pueden absorber, por ejemplo, parte de la radiación infrarroja (las moléculas de agua y de dióxido de carbono absorben en el infrarrojo) o del ultravioleta (el vidrio bloquea parte del espectro ultravioleta). La transparencia se cuantifica como transmitancia, porcentaje de intensidad lumínica que atraviesa la muestra. Para esto se utiliza un colorímetro o un espectrofotómetro.

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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 super-set 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. 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. 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 back 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. Transparency can provide almost perfect camouflage for animals able to achieve it. This is easier in dimly-lit or turbid seawater than in good illumination. Many marine animals such as jellyfish are highly transparent.

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