Clear, transparent, translucent, lucid, pellucid, diaphanous, limpid are comparable when they mean having the property of being literally or figuratively seen through.
Something is clear which is free from all such impediments to the vision as clouds, mist, or haze (clear air) or from muddiness, cloudiness, or turbidity (clear glass) (clear crystals) or from obscurity, vagueness, or indistinctness of any sort (clear thinking).
Something is transparent which is either so clear or so thin that objects can be easily seen or perceived through it.
Something is translucent which admits the passage of light through it but which does not permit a clear sight of what lies beyond.
Something is lucid (see CLEAR 2) which is both transparent and luminous; this use is chiefly literary.
Something is pellucid which is clear as crystal.
Something is diaphanous which is so delicate and gossamerlike in texture that it is almost transparent or is actually translucent.
Something is limpid which has the soft clearness of or as if of pure water.