Why is literature a prism?

A prism is clear, like a window, so you can see through it. But unlike a window, it distorts the light, separating it into colors. So what you're seeing is not the true picture, but an altered one.

In the same way, literature provides us with a way to look at another time (Vanity Fair), a different culture (Things Fall Apart), or even a hypothetical future (Brave New World). But the way the author chose to present the setting and characters will 'color' our view of that period or society; so in that sense literature is more like a prism than a window.