These words are antonyms but for some reason are not always readily distinguishable. Opaque means "not transparent," "not allowing light to pass through," "not bright," "dull," and "dark": "This thickly woven screen is opaque." "Despite your help, the problem still seems opaque to me." Transparent has an exactly opposed meaning. A transparent substance permits light to pass through, can be seen through; transparent also means "frank," "open," and "obvious": "Clean water is transparent." "His eagerness to accept the offer is transparent." Words related to transparent are clear, pellucid, crystalline, limpid, and translucent.
Dictionary of problem words and expressions. Harry Shaw. 1975.