In order to see an object, light from the object must enter our eyes. We can see stars, lightning, and light bulbs because they are emitting or giving off light. We depend on the light emitted from these sources in order to see objects that don’t emit light—we see those objects because they reflect light into our eyes. The paper on which this book is printed, for example, does not emit light. We see it because the paper reflects light into our eyes.