Light and dark are symbolic powerhouses in art. Bad things lurk in the shadows while angels are surrounded by halos; Darth Vader dresses in black from head to toe while brides beam in all white. Are you noticing a trend? Yup; usually we associate light with goodness and dark with all things ominous. Things are anything but usual in Tomorrow, When the War Began, though,and so are light and dark.
The darkness in this story is where the kids can hide safely. All of their spying and sneaking operations take place in the dark—it's their veil, the cover that protects them. As the teens sneak around the Showground, Ellie says, "To come out of the darkness now would be to show courage of a type that I'd never had to show before […]" (7.64). For these kids, the light is frightening because it reveals them to the soldiers who have the power to destroy them. Because of this, they do all their travel at night.
Light plays another role in the book, too: It's also transformative. When Ellie sees the Showground and soldiers in all that light, she can't help but acknowledge the truth that her country has most definitely been invaded. And when she chooses to step into the light anyway, it's a moment that draws a hard line between who she used to be and who she's not becoming:
A small single movement was my key to finding my spirit. […] four quick light steps, a dance that surprised me […] I was transformed by those four steps. At that moment, I stopped being an innocent rural teenager and started becoming someone else, a more complicated and capable person, a force to be reckoned with even, not just a polite and obedient kid. (7.65)
Ellie's steps into the light mark her shift from "innocent" to "capable person," someone who can and does fight back against those with dangerous life-threatening weapons in order to try to get her family back. As much as the dark conceals her and keeps her safe, then, it is also a way of hiding from the truth and not being her fullest self. In this regard, light and dark play more classic roles as symbols. Boom.