As this is about speed of light, reminds me of a school project i did back in Kenya, year was 1972, and in Kenya it was not the easiest thing to get hold of components, there was only one shop selling some parts, if any at all, and my knowledge of electronics was very very limited, I did not know that i could have used two 4.7K resistors in series to create a 10k ohm resistor or that I could use two or more other value resistors to create a different value, in some projects and instances the value is not always hyper critical at all.,
But nevertheless I made a project for my school that won me an award amongst all the top schools in Nairobi, I made project that used a beam of light to transmit an audio signal at a distance, similar to todays fibre optics!
so I had on one end a small torch bulb that lit from a source of audio output from a small cassette tape recorder, such that its intensity varied with music or speech, however, the small torch bulb 2.5V would not light up below 1 volt of audio so I had to pass a dc current constantly to keep it just light, this is called bias, this removed all distortion from the audio output as on very soft notes the bulb would not produce any light variation, so by biasing it constantly with a dc current, the bulb remains partially lit, except the audio signal now modulates its intensity, making it go brighter and darker, the demonstration model I made worked at d at a fair distance like 100 meters, using just a small bulb and a 2watt audio output.
The bulb was placed in front of a concave reflector such that it would project a parallel beam as opposed to a diverged beam, i also used a large 3" concave lense to trim the beam absolutely parallel.
At the receiver end I had a similar set up where I used another concave lense to focus the light beam on to a transistor, in those days OC44 was a general purpose germanium transistor packaged in a glass capsule and coated with black paint to keep the light out, so if you scrape off the black paint and focus the light on to its junction, the transistor acted as a photo cell and so when you pass a small dc current through it, the current is then modulated by the incoming photo beam, such that this minute variation in the photo beam controls the flow of the current through the germanium transistor's collector emitter junction. and by using a few other components, this supplies enough amplification to be fed into a phono input of another record player where the sound is then amplified to drive a loud speaker!
the exact sound is reproduced that is playing at source, quite a few meters away, place your hand in the beam and the sound stops, sadly this project was far too advanced and beyond the ability of even my school physics teacher! He was totally puzzled, he though it was magic!
Even the judges were baffled, and did not really understand why anyone would need to make such a project, and what it would be the use of such an idea, but still they gave me a second price, and guess who won the 1st price, a couple of girls who made a hoover craft out of polystyrene and installed a small dc electric motor to fan some air through its skirt and it floated, but used an external power supply which if you really mounted on the craft itself it would never lift up! but they won 1st price!
41 years ago, My project was far too advanced for those judges ability to understand how light can transmit signals, and today we have Fibre Optics in use taking over from copper cables, and hoover craft is now in science museum!
