What is the light and dark imagery in A Tale of Two Cities?
Light and dark imagery is so pervasive in A Tale of Two Cities that it would almost be easier to list chapters where Dickens doesn't draw on the motif. In addition to the passages other responses have noted, however, we might also look to the first few sentences of the novel for clarification.
Dickens opens with the very famous line, "It was the best of times, it was the worst of times," followed by a chain of similarly paradoxical statements—including, "It was the season of Light, it was the season of Darkness" (Book 1, chapter 1). This, then, is a major clue as to how Dickens will use light and dark imagery throughout the rest of the novel: A Tale of Two Cities centers on pairs of opposites—London vs. Paris, Carton vs. Darnay, etc.—and Dickens will use light and darkness as a way of developing each of these pairings. This is likely most obvious in his treatment of Lucie and Madame Defarge, who are not only moral but physical opposites: Lucie's bright golden hair—a symbol of hope to her father—contrasts strongly with Madame Defarge's "darkly defined eyebrows" (Book 1, chapter 5).
With that said, as the novel progresses, the binary distinction between light and darkness begins to break down—most obviously via the character of Sydney Carton, whose cynical and dissolute exterior hides true nobility and selflessness. As he is (presumably) contemplating sacrificing his life for Darnay's, Carton is a figure of both light and shadow: "A light, or a shade (the old gentleman could not have said which), passed from [Carton's face] as swiftly as a change will sweep over a hill-side on a wild bright day" (Book 3, chapter 9). Dickens will go on to depict Carton as a Christ figure, lending additional depth to the novel's use of light and darkness by tying it to themes surrounding resurrection. In the following passage, for instance, Carton views the coming dawn as analogous to life triumphing over death:
The night wore out, and, as he stood upon the bridge listening to the water as it splashed the river-walls of the Island of Paris, where the picturesque confusion of houses and cathedral shone bright in the light of the moon, the day came coldly, looking like a dead face out of the sky. Then, the night, with the moon and the stars, turned pale and died, and for a little while it seemed as if Creation were delivered over to Death’s dominion.
But, the glorious sun, rising, seemed to strike those words, that burden of the night, straight and warm to his heart in its long bright rays. And looking along them, with reverently shaded eyes, a bridge of light appeared to span the air between him and the sun, while the river sparkled under it. (Book 3, chapter 9)
Ultimately, then, Dickens uses light and darkness to underscore the possibility of rebirth, not just on a personal or moral level (as in the case of Carton or Dr. Manette), but also on a societal one; the narrator speculates that in the moments before his death, Carton foresaw "a beautiful city and a brilliant people rising from this abyss, and, in their struggles to be truly free, in their triumphs and defeats . . . gradually making expiation for [the 'evil of this time']" (Book 3, chapter 15).
Light and dark imagery is used throughout the novel. From the very first chapters we are introduced to a shadowy, dark, scarry world - the mist and darkness enshrouding the mailcoach is a perfect example. The appearance of Lucie also perpetuates this imagery, as the darkness in the room is described as being so strong that the light from the candle offers little respite.
Shadows and darkness continue to fall across the rest of the novel. In particular, Madame Defarge casts a shadow on Lucie and her hopes of freeing her husband. For example, as Lucie stands in the fresh, white snow, Madame Defarge passes like a "shadow over the white road". Likewise the letter written by Dr. Manette casts a shadow over the whole family. It is highly significant that the chapter in which the letter is read out is entitled "The Substance of the Shadow".
Against this darkness light is represented through the character of Lucie, and in particular "The Golden Thread" of her hair.
There are a number of dark instances being used starting in the first chapter where the author speaks of all the criminals reigning terror on the people to such an extent that it all seems normal. He ends this point by talking of the man who is ever so busy hanging criminals one after another, sometimes in groups, every single day: “hanging a housebreaker on Saturday who had been taken on Tuesday; now, burning people in the hand at Newgate by the dozen.” The steaming mist and darkness that looked like an “evil spirit” also goes to show some level of dark imagery. In the third chapter light is seen with the rising of the sun which was bright, placid and beautiful “though the earth was cold and wet, the sky was clear, and the sun rose bright, placid, and beautiful.”