180. How does a heat-seeking missile and a radar-homing missile work?
A heat-seeking missile studies the infrared light coming toward it from the sky in front of it. It uses a lens to form a real image of that light on an array of infrared sensors. If there is a hot object in front of the missile, that object will emit more infrared light than its surroundings and the missile's lens will form a bright image of the hot object on one of the infrared sensors. If the bright image falls on the central sensor, the missile will do nothing—it will flight straight ahead. But if the bright image falls on one of the side sensors, the missile will turn. It will turn by deflecting its rocket exhaust so that the missile begins to rotate in flight. As the missile rotates, the image of the hot object will move from one sensor to the next and it will eventually fall on the central sensor. At that point, the missile will stop turning and will flight straight ahead. Since the missile automatically turns to head toward the hot object, it will eventually fly right into the hot object and explode. A radar-seeking missile will do that same things, except that it will look for an object that is emitting lots of microwaves (radar), rather than lots of infrared light. A radar-guided missile is much more complicated, since it must first emit a burst of microwaves and then analyze the reflected microwaves to look for something to fly toward. Many laser-guided missiles are just like heat-seeking missiles except that they look for an object that is reflecting a laser beam. The people who fire the missile simply illuminate the target with a bright laser beam and the missile flies directly toward the laser spot on the target.