This window-pane is transparent.

Transparent : (of a material or article) allowing light to pass through so that objects behind can be distinctly seen.
Opaque : not able to be seen through; not transparent.
Translucent : (of a substance) allowing light, but not detailed shapes, to pass through; semi-transparent.
Solid : firm and stable in shape; not liquid or fluid.
Fragile : (of an object) easily broken or damaged.

Antonym of Transparent is Opaque