Although we all think of morse code as a method of communication using sound, it is Not just limited to sound! Morse code can also be sent by light or by tapping on a surface, this makes decoding even more harder although sending by light using devices such as ALDIS lamps has been used since the late 1800's. Mainly between ships.
Was even used in the first world war when communications were out.
It has its limitations though... speed is generally limited to no more than 12wpm and takes great skill and practise to decode easily!
Can also be used to preserve Radio Silence.
In case you were wondering, The mesage by flashlight in the corner reads:
'MORSE WITHOUT RADIO'
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