The missing piece here is that a given signal only reach the next device in the chain. Then this device emit a new signal -which can be optical, electrical, or radio- to communicate with the next device.

Think about the alarm tower in the Lord of the Rings: a fire is lit at Minas Tirith, which is seen by a watcher on a mountain peak. The watcher lit a second fire, which is seen by a third watcher, and so on on. At the end of the chain this is a human that emit a vocal signal that reach the other peoples.

Only the first watcher can see the original signal, and there's different transmission method used (light from the fire and voice).

The missing piece here is that a given signal only reach the next device in the chain. Then this device emit a new signal -which can be optical, electrical, or radio- to communicate with the next device.

Think about the alarm tower in the Lord of the Rings: a fire is lit at Minas Tirith, which is seen by a watcher on a mountain peak. The watcher lit a second fire, which is seen by a second watcher, and so on on. At the end of the chain this is a human that emit a vocal signal that reach the other peoples.

Only the first watcher can see the original signal, and there's different transmission method used (light from the fire and voice).