Throughout his narrative in Joseph Conrad's Heart of Darkness, Charlie Marlow characterizes events, ideas, and locations that he encounters in terms of light or darkness. Embedded in Marlow's parlance is an ongoing metaphor equating light with knowledge and civility and darkness with mystery and savagery. When he begins his narrative, Marlow equates light and, therefore, civility, with reality, believing it to be a tangible expression of man's natural state. Similarly, Marlow uses darkness to depict savagery as a vice having absconded with nature. As Marlow continues with his story, he realizes that civilization and inhumanity are very close to one another. The metaphor that he has been using for light with civility and darkness with mystery transforms up to the point where Marlow has a realization of where the "light" reality of society is covered by the immense "darkness." In the end of the novel, Conrad notes that the Thames "lead[s] into the heart of an immense darkness."
The theme of dark and light realities continues in the narrative when Marlow associates intelligence and wisdom as "light dawned upon" him. Marlow also believes in this book that light and whiteness/chivalry/bravery is associated with society and civilization. Marlow, when beginning the story, speaks of the Europeans who left the Thames River to occupy far off places. He notes that they brought "light into the darkness" and, therefore, makes the connection of light and society.
Further along in the story, Marlow begins to realize that civilization (especially how he referred to it in the conquering sense) is really not very light at all, and is really a dark reality. When Marlow first arrives in the Congo, we walks through the shade and the light, and meets the black slaves (who are dying) in darkness and the white workers/management in the light.