A periscope is a useful example of the law of reflection at work. It's important that your mirrors are placed at a 45° angle, because light always reflects away from a mirror at the same angle that it hits it. In a periscope, light from an object strikes the top mirror at 45° and bounces off at the same angle. This sends light directly down the tube and onto the lower mirror. This mirror, also at a 45° angle, reflects light directly to your eye.