Transparency (optics)
In
optics,
transparency is the property of allowing
light to pass. Transparent materials can be seen through; that is, they allow clear images to pass. The opposite property is
opacity.
Translucent materials allow light to pass through them only diffusely, that is, the material blurs the image.
Though transparency usually refers to visible light in common usage, it can actually refer to any type of radiation. For example, flesh is transparent to
X-rays, while bone is not, allowing the use of
medical X-ray machines.
Examples of transparent materials are
air and some other
gases,
liquids 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 for instance 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 a thick smoke.
From
electro-dynamics it results that only a vacuum is really transparent in the strict meaning, any matter has a certain absorption for
electro-magnetic waves.
There are transparent
glass walls that can be made
opaque by the press of a button, 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.
Examples of translucent materials are frosted glass,
paper, and some kinds of
amber. Opacity is based either on absorption or on reflection of the light falling onto the material.
*
Transmittance*
Electromagnetically induced transparency*
Transparent alumina*
Zone plate*
List of optical topics
