Near the port of Alexandria there is a fine high tower, on which there was not long ago a mirror, in which one could see from Alexandria toward Cipern those who were on the sea; and whatever they were doing, all could be seen in this mirror at Allexandria, so that at the time that the king of Zipern went to war with Allexandria, he could do them no harm. Then came a priest to the king of Ziperen, and asked what he would give him if he broke the mirror. The king replied, that if he would break the mirror, he would give him whichever bishopric he might choose to have in his country. The priest then went to Rome to the Pope, and said: That he would break the mirror at Allexandria, if he would allow him to abjure the Christian faith. He gave him permission that he might do so in words, and not in deeds nor with the heart. Now he did this for the sake of the Christian faith, because the Christians at sea suffered many injuries from the Infidels, through this mirror. The priest returned from Rome to Alexandria, and was converted to the faith of the Infidels, and learnt their writing, and became an Infidel priest and their preacher, and taught them the Infidel faith against the Christian faith, and they held him in great honour, and wondered, because he had been a Christian priest, and they trusted in him very much. They asked him which temple in the city he wished for, as they would give it to him for his life time. There was also a temple in the middle of the tower where the mirror was; this temple he asked for, for his life time; they gave it to him together with the keys of the mirror. There he remained nine years, and then one day he sent to the king of Zypperen that he should come with his galleys, and he would break the mirror which was in his power, and he thought, that, after breaking the mirror, if the galleys were there, he would go on board. One morning many galleys came, he struck the mirror three blows with a hammer before it broke, and from the noise all the people in the city were frightened, and ran to the tower and fell on him, so that he could not get away; then he jumped out of a window of the tower, into the sea, and was killed.
In the (extensive!) footnotes one finds:
Makrizi describes the pharos at Alexandria (S. de Sacy, Chrestom. Arabe, ii, 189) as having at the top a large mirror, around which criers were seated. Upon perceiving the approach of an enemy through the agency of this reflector, they gave warning to those in the immediate neighbourhood by loud cries, and flags were displayed to apprise others at a distance, so that people in all parts of the city were immediately on the alert.
I don't know about you, but the first thing that comes to my mind with mirrors and lighthouses is "optics." "Consider that Bacon, in the fifth book of the Opus maius, waxed enthusiastic about the Ancient's ability to enlarge small objects and to bring faraway ones close, using appropriate configurations of lenses and mirrors."
The use of lenses has been known since antiquity. This is in Alexandria, and the earlier references to it date back to the 8'th century. It sounds a lot like a telescope.
Although this wonderful "mirror" is also supposed to have been used to set ships on fire by reflecting the sun's rays--sorry, not big enough for that. But little demonstrations inside the tower might have been frightening.
And ...., it turns out my speculation isn't orignal. That article includes a truly spectacular speculation about a telescope in the lighthouse.