Imagery associated with light and dark is a motif in the novel. Often, dark heralds danger and evil, while light brings reprieve from these things. There is an exemplary interplay between light and dark in Chapter 13, when Bilbo and the dwarves find the door out of the Lonely Mountain:
A misty sun sent its pale light between the arms of the Mountain, and beams of gold fell on the pavement at the threshold.
A whirl of bats frightened from slumber by their smoking torches flurried over them; as they sprang forward their feet slithered on stones rubbed smooth and slimed by the passing of the dragon. Now before them the water fell noisily outward and foamed down towards the valley.
The light of the sun is not particularly strong. "Pale" and filtered by mist, the sun's rays ought to be lackluster. However, to the hobbit and dwarves who have been stuck inside the dark mountain lair of a dangerous dragon, the sunbeams have all the majestic splendor of "gold." The sight of the sun is as valuable as any real gold they might have found within the mountain. Under its light, by way of the waterfall, they will still have to walk on slippery stone. This stone, however, will feel different beneath their feet than the "slimed" stone over which the dragon has "slithered" before them. It is wet instead from the "noisy," "foaming" water that comes together with the sun to render the valley below fertile. The feel of the wet stone outside the mountain is associated with the promise of life and abundance Smaug long ago extinguished inside the mountain.
Light imagery is not always as straightforward as this. Earlier in the chapter, when Bilbo finds the Arkenstone, Tolkien plays with light imagery to emphasize the intense temptation the hobbit feels:
Ever as he climbed, the same white gleam had shone before him and drawn his feet towards it. Slowly it grew to a little globe of pallid light. [...] At last he looked down upon it, and he caught his breath. The great jewel shone before his feet of its own inner light, and yet, cut and fashioned by the dwarves, who had dug it from the heart of the mountain long ago, it took all light that fell upon it and changed it into ten thousand sparks of white radiance shot with glints of the rainbow.
The way the Arkenstone refracts light is beautiful, but it is also overwhelming. Its "inner light" suggests that it is powerful and that it has the capacity to light up any dark space—that is, to be used for good. At the same time, all the light that shines on it is "changed...into ten thousand sparks of white radiance shot with glints of the rainbow." Bilbo's good nature is a kind of "light" in itself that he shines upon the Arkenstone. The stone bends and scatters this light so that Bilbo's innate goodness is no longer angled in a clear direction. He becomes confused about exactly what to do with the stone, and he ends up hiding it until he realizes that he can use it for diplomacy. Even then, he is not sure he is using the stone purely for good, especially given that he must betray Thorin to carry out his plan. The stone's ability to refract light represents the challenge of harnessing great power for good. Even the best-intentioned people can be tempted, corrupted, or simply befuddled in the face of a powerful object like the Arkenstone.
In Chapter 14, Bard kills Smaug by shooting him in the one spot on his belly that is not encrusted with protective gems. Tolkien uses imagery to emphasize Smaug's weak spot as an allusion that connects The Hobbit to the epic tale of the Trojan War:
The dragon swooped once more lower than ever, and as he turned and dived down his belly glittered white with sparkling fires of gems in the moon—but not in one place. The great bow twanged. The black arrow sped straight from the string, straight for the hollow by the left breast where the foreleg was flung wide. In it smote and vanished, barb, shaft and feather, so fierce was its flight. With a shriek that deafened men, felled trees and split stone, Smaug shot spouting into the air, turned over and crashed down from on high in ruin.
In The Iliad, the famous epic poem that tells the story of the Trojan War, a hero named Achilles is the most formidable warrior of the Greek army that invades Troy to reclaim their stolen queen, Helen. Achilles is such an impressive fighter not only because he is highly skilled, but also because of a blessing he received as a baby. Supposedly, his mother (a sea nymph) held him by his heel and dipped him in the River Styx. The water gave him a kind of everlasting magical armor, so that he could never be injured. The catch is that the heel his mother held onto during the ritual did not receive the same protection. Details about Achilles's death do not appear in The Iliad, but other sources complete the story: after Achilles kills the Trojan prince Hector, Hector's brother Paris shoots a fatal arrow through Achilles's vulnerable heel.
Like Achilles, Smaug is a seemingly invincible enemy, both because he can do a lot of damage and because he has the best armor imaginable. Smaug's armor is not explicitly magical, but it does have an air of mysticism and symbolism. The image of "sparkling fires of gems in the moon" underlines the way Smaug has stolen the glittering treasure in the mountain, fused it onto his skin, and turned it into a fiery coat of armor that is powerful enough to light up the night. In the Trojan War, the Greek army (for which Achilles fights) sneaks into Troy inside a great wooden horse to begin a 10-year military occupation. Likewise, Smaug has snuck into the Lonely Mountain for a long occupation. It is up to the mountain's inhabitants to, like Paris, find the conviction and means to fight this formidable enemy off.
The "deafening" imagery of Smaug's intense shriek further reinforces the epic scale of The Hobbit, even though it is essentially a children's story. The consequences of this battle—felled trees, split stone—are legendary. By alluding to Achilles's weak heel, Tolkien encourages the reader to see other parallels between this book and ancient Greek epics. For instance, Bilbo's constant teetering between adventurousness and homesickness makes him a lot like the hero Odysseus. Tolkien's use of songs emphasizes the novel's connection to the kind of oral storytelling that these ancient epics are based on. Tolkien thus hints at a rich mythology and history surrounding The Hobbit, even though it all takes place in a little world of his own invention.
In Chapter 17, Gandalf arrives with news that Bolg, a new goblin leader, is leading the goblins and wargs into battle with the dwarves, men, and elves. As Bolg advances, Tolkien describes the atmosphere using imagery and alliteration:
Still more suddenly a darkness came on with dreadful swiftness! A black cloud hurried over the sky. Winter thunder on a wild wind rolled roaring up and rumbled in the Mountain, and lightning lit its peak.
The thunderstorm's darkness is soon amplified by a cloud of bats that flies in to fight on behalf of Bolg. Tolkien does not simply tell the reader that there is a storm, but moreover emphasizes the sensory experience of standing in a storm as it begins. The sky turns dark so fast it is "dreadful," like an omen of terrible things to come. Darkness typically represents danger in this novel, whereas light represents reprieve. The sudden deprivation of light in the middle of the day suggests that there is no longer anywhere for the protagonists (especially Bilbo) to hide from the dangerous enemies they have accumulated along their way to the Lonely Mountain. Tolkien emphasizes the unusually cold wind for a thunderstorm, as well as the deep, "roaring" sound of the thunder that closes in on the mountain. The sound is not dissimilar to Smaug's "roar." The dragon may be defeated, but the sound signals that the adventurers have yet to face the greatest danger of all. Tolkien also describes a flash of lightning, which lights up the sky but also raises the hair on the back of the reader's neck. This is not the kind of light that saves anyone from the dark, but rather the kind of light that might set the whole place on fire. Again, there is the sense that Smaug's presence still lingers over this place.
Tolkien uses alliteration to make all this imagery even more effective. The "darkness" is "dreadful," "sudden," and "swift." The "wild wind" brings "winter thunder" that "roll[s] roaring up and rumble[s]." The "lightning [lights]" up the top of the mountain. Some of this alliteration is onomatopoetic and transports the reader into the scene. The repeated "r" sounds echo the drawling sound of thunder even as the words describe that sound. Spoken aloud, the repeated "w" sounds require the reader to recreate the sound of wind by forcing air through their mouth. Even the alliteration that is not onomatopoetic makes for a sense of breathlessness that helps build tension and signal that the the coming battle will be monumental to the plot and to Bilbo's character development.