When we were children we used to play a game in which a mirror was used to reflect the sun's rays upon the wall. The ray of light perhaps conveyed a secret message understood by the people playing the game, a type of optic communication.
我們小時候常玩的一種遊戲：用一面小鏡子將陽光折射到牆上，打出與玩伴約定的一閃一滅的訊號來秘密通信，就是一種光通訊。