Trying to measure the speed of light - part 1.

One of my current 'projects' is to measure the speed of light - directly, by timing how fast it goes from A to B. The first to do this were Fizeau and Foucault, with two slightly different methods, around 1850.  The principle of Foucault's experiment is that the beam travels between two mirrors, and that one of the mirrors has rotated a little in the meantime. This animation shows nicely how this works:

Kevin McFarland, University of Rochester

Later Michelson and Morley further refined the method, in the hope of proving the existence of aether for once and for all. Their experiment is one of the most famous failed experiments of all time, indirectly leading to the birth of relativity theory.

Today I made my first steps towards measuring how fast light travels. My first try was to use an oscilloscope to measure time differences. The idea is to split a beam in two, and send both to phototransistors, but one directly and the other over a distance of, say, 100 meters. 100 meters divided by 300.000.000 meters per second equals 0.3 microseconds, which is easily within measuring range of current oscilloscopes.

I also hoped to use, for instance, my trusted Logitech laser pointer, the one that I use for all my presentations, and which the kids use to play with the cats. It has a nice clicky button, and my plan was to use the button to turn on the laser, which then gets split and travels through different paths to the phototransistors. Unfortunately, it turns out that although the laser turns on relatively fast, it's not fast enough:

As you can see, the rise time of the signal is about 40 microseconds. I'm not going to be able to recognize a time lag of 0.3 microseconds in a 40-microsecond-ramp, that will disappear in the noise. Hmm, this is not yet right.

It turns out that the response time of the phototransistor I'm using is 15 microseconds, so the slow increase is probably due to the laser itself. Since lasers can be much faster, maybe I can find a way of hacking a laser pointer to fire up quickly. With a faster phototransistor I might be able to get somewhere. 

Unfortunately, it seems there is another problem: my estimate of 100 meters is somewhat optimistic. The Logitech's beam fans out over 20 meters to a splash of light, about 10 cm wide, with a core of about 3 cm. Over 100 m that will be 15 cm, which makes me worried about being able to detect anything ...

Lots to think about, then ...