H22a: Flashing Light in Lighthouses
Q: Lighthouses flash. Why is that?
Think of this experiment. Wait until after dark, then get yourself a torch and switch it on. Turn out all the lights in the room and hold the torch parallel to the floor so that it shines out ahead of you. Ask a friend to stand behind you, facing the same direction as you. Now turn around to your right, without moving from your position on the floor, and turn a full circle so that you finish up looking in the same direction you started from. What does your friend see? She sees a beam of light coming around from her right and going off to the left. The light is bright when it shines at her, but loses brightness when it has gone past.
Being close to you, your friend can see the light from the torch even when it is not pointed at her because you are indoors and the light gets reflected off the walls and furniture. If you were both outside in the countryside - or in the middle of the sea (in a boat!) - your friend would see nothing when the beam is pointing away from her. As the beam turns around to your friend's direction, it would appear to get brighter, reach a peak brightness, and then get less bright until it disappeared once more. So what your friend would see is a flash. But the light is on all the time. It is just formed into a beam and rotated. If you hold one torch and take ten seconds to turn around, your friend will see one flash in ten seconds. If you hold two torches pointing in different directions, your friend will see two flashes in ten seconds. Or you could take fifteen seconds to turn around...
So you see that lighthouses hold different numbers of torches and take different amounts of time to turn around. That's what gives them each a flashing code that we call a characteristic.