Transparent glass ball

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.

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 and Pyrex. 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.

Translucency

A view through a translucent material is blurry or distorted. Examples of translucent materials are frosted glass, paper (tracing paper), and some kinds of amber. It can sometimes be difficult to identify translucent objects. For example, while it is easy to see the fur of a Polar Bear is white, it takes a truly close look to learn it is also translucent. Opacity is based either on absorption or on reflection of the light falling onto the material.