The relationship between the words "radiant" and "somber" is that they are antonyms, with "radiant" meaning bright and cheerful, and "somber" meaning dark and gloomy. In the same way, the words "ample" and "scant" are also antonyms, with "ample" meaning plentiful or abundant, and "scant" meaning lacking or inadequate. Therefore, the analogy "radiant : somber :: ample : scant" is a valid analogy as both pairs of words exhibit the same relationship of being antonyms.