Windows that can't be broken and can switch from opaque to clear.
Crystals of barytes may be transparent and colourless, or white and opaque, or of a yellow, brown, bluish or greenish colour.
When light proceeding from a small source falls upon an opaque object, a shadow is cast upon a screen situated behind the obstacle, and this shadow is found to be bordered by alternations of brightness and darkness, known as " diffraction bands."
In an engraved glass grating there is no opaque material present by which light could be absorbed, and the effect depends upon a difference of retardation in passing the alternate parts.
Another page informed her that the opaque green berries on the thorny vines were gooseberries.