In optics, transparency is the property of being transparent, or allowing light to pass. The opposite property is opacity. 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.

Transparent materials can be seen through; that is, they allow clear images to pass. Translucent materials allow light to pass through them only diffusely, and hence cannot be clearly seen through. Examples of translucent materials are frosted glass, paper, and some kinds of amber.

There are transparent glass walls that can be made opaque by the press of a button, a technology known as electrochromics.

Transparent or semi-transparent clothing enables play with the boundaries of dress-codes regarding modesty; one example is the wet T-shirt contest.

For derived uses of the idea of transparency.