An antonym is a word which means the opposite of another word. Antonyms are also called opposites. One way of mastering a language involves learning its vocabulary. Synonyms and opposites are helpful in this sense
Two opposites of "light" are "dark" and "heavy".
Here is a list of opposites arranged in alphabetical order.
NOTE: this list of opposites is of course by no means an extensive one. More opposites can be added.
Related material
List of adjectives.
List of opposites (adjectives)
List of Synonyms
Exercise on opposites