The diagram shows light interacting with an object. A blue box has 3 black arrows pass from left to right to its left surface, which they strike at a shallow angle. The arrows then become lines, which move through the box at a different angle. The arrows reemerge as arrows from the right side at the same angle at, which they first struck the box.

Which object is the light most likely striking?

a window
a mirror
a plaster wall
a steel beam