Fundamentals'ish

So starting out we need to know a few basic fundamentals about the experiments I am wanting to perform, and the limitations of what can be done.

The one fact we know - the SI unit for c in a vacuum is 299'792'458 m/s

We will be sending a pulse of light from a source to a mirror some distance away and attempting to measure the time it takes to complete the round trip. This is the Time Of Flight (ToF) method used for over a hundred of years.

What I have at my disposal is a Rigol DS1102D 100MHz - 1GSa/s oscilloscope that is somewhat limited by the stock 60MHz bandwidth probes.

I have a mirror of unknown origin sellotaped to the fireplace 20 metres away. Your getting the idea.

I have a (apparently fancy) 532nm 150mW laser diode that I kind of splurged on with out thinking. That will be the bane of my life, I'll discuss later. It has a TTL input that I have connected my trusty Raspberry PI's hardware PWM output to.

The PWM output is the trigger of my oscilloscope channel 1 and the TTL input of the laser. I have constructed a photodiode detector that connects to channel 2.

I will talk about all these things in more detail in future posts.

An interesting question that never gets talked about because apparently it's just 'so obvious' is why do all these experiments use a mirror to return a pulse back to the source? Well it's very hard to synchronize two clocks at a distance to the accuracy we need. If we had a detector 20 metres away and sent a signal back to our single clock then how would we transmit that information? copper wires are slower then the light we are sending...

We will be working at around 1 atmosphere so need to adjust our value of c for that by taking in to account the refractive index of air. That's 1.000293 at 1 atmosphere at 0 degrees centigrade. Getting a number for room temperature at higher pressures and at my lasers specific wavelength looked like a whole world of hurt after a little googling so I will defer that and let the number lie for now.

Our corrected c_atmos = 299'704'645 m/s. A whole 87'813 m/s slower - That's HEAPS!

That means over my 40 metre distance I would expect the ToF to be 133.464732 nano-seconds.

Notwithstanding the finer details of oscilloscope bandwidth limitations I'm just going to take the reciprocal of our 60MHz probe to be the worst case in the system and say that the minimum time I could measure is 16ns and worry about the detail later.

So our error will be between 133ns and 149ns. That is ONLY a 32'082'982m/s error.. HEY were within the 11% range I can live with that for a first attempt.

Well, lets just suck it and see.