...and that sine wave would have to be...well...10cm min distance so presume we are talking resolutions of around a 1mm...let's see what is it? Err.. something times something-or-other equals the velocity of light...hhmmmm...they must be able to modulate optos at a awful lot higher frequencies than when I used to play with them.

Of course that was aeons ago - well before the Intel 4004. Great chip that. In fact I think they are still being made - except they call them PICs now. :-)