Transparency (optics)

In optics, transparency is the material property of allowing light to pass through. In mineralogy, another term for this property is diaphaneity. The opposite property is opacity. Transparent materials are clear: they can be seen through. Translucent materials allow light to pass through them only diffusely: they cannot be seen through; contrary to popular belief, translucency does not include see-through colored objects such as (for instance) emerald in its cut state (which is transparent) but does include things such as frosted glass which allow light to come through but no images.

Transparency
Though transparency usually refers to visible light in common usage, it may correctly be used to refer to any type of radiation. For example, flesh is transparent to X-rays, while bone is not, making X-ray imaging useful for medicine.

Examples of transparent materials are air and some other gases, liquid such as water, most glasses, and plastics such as Perspex. Where the degree of transparency varies according to the wavelength of the light, the image seen through the material is tinted. This may be due to certain metallic oxide molecules in glass, or larger colored particles, as in a thin smoke. If many such particles are present the material may become opaque, as in thick smoke.

From electrodynamics it results that only a vacuum is really transparent in the strict meaning, any matter has a certain absorption for electromagnetic waves.

There are transparent glass walls that can be made opaque by the application of an electric charge, a technology known as electrochromics.

Certain crystals are transparent because there are straight lines through the crystal structure. Light passes unobstructed along these lines.

There is a complicated theory "predicting" (calculating) absorption and its spectral dependence of different materials. See: absorption (optics) - absorption of photons by a material and absorption spectroscopy.