what about eps_0 and mu_0, constants permeability of magnetic and permittivity of electric space? light is an EM wave, and its velocity is magically equal to 1/sqrt(eps_0 mu_0).

Would be better off sending out a FM radio wave and mixing on the return, then measuring beat frequency vs input frequency to determine distance, sorta like a doppler radar except the measured variance is from the emitted FM signal rather than actual doppler effect.

I agree.. Capacitor should work. 2 sensors. Use a comparator that charges the cap when the outputs are different. The difficulty will be balancing the inputs.

Well, actually the speed of light in the optical fiber is quite a bit less. The index of refraction of most glass is about 1.5 Therefore light is moving at 1/1.5 or 67% of the speed of light in a vacuum. Lenses focus light because in the thick part of the lens, the light spends more time at the reduced speed, therefore emerging from the lens at a later time than light from the thinner part, so the beam bends toward the slow (thick) side. Electrical pulses in a copper wire move at about half the speed of light, or about a foot a nanosecond. Even the atmosphere around us makes light slow to 99.97% of C. Even the dust in outer space slows light from distant super novas enough so that it arrives at earth several days later than the neutrinos from the event do. Turns out that it isn’t all that difficult to put the brakes on light. Or is that put the brake lights on?

Actually, it should be quite easy just with one detector, one mirror; the laser and an Arduino running at 16MHz.

Firstly you get the laser to generate PWM pulses of light controller by the Arduino. The frequency isn’t very important; something in the order of ms will be fine.

You set a timer-capture unit so that it’s at its maximum supported frequency (8MHz on a 16MHz Arduino) and measure the period between when the pulse is sent and when it’s detected.

You move the mirror to a distance so there’s at least 1 clock pulse between starting the laser and receiving the result. The mirror should be placed at the minimum distance where this is the case: Distance m0, resulting in p 8MHz clocks.

You then move the mirror to the minimum distance where there’s p+1 8MHz clocks between sending a laser pulse and detecting it. This is distance m1.

The speed of light is: (m1-m0)*0.5*8000000m/s.

m1-m0 will ideally be 18.737m; a little shorter when measuring the speed of light in air; but still, a feasible experimental distance (m0 will add at least another 18.737m).

On a 20MHz AVR-based computer, such as a FIGnition, this would be reduced to 15.5m: even more feasible :-)

In either case, the upshot is that you don’t need nanosecond resolution, nor complex apparatus!

-cheers from julz

[Note: obviously I'm working in metric, m is in metres]

There are a couple of half-decent ways to measure very accurate time periods.

The first is a cascaded row of logic gates (like an asynchronous shift register). Use the “start” pulse to send a signal into the first gate, that propagates through each gate in turn until the “stop” pulse comes along and disables the propagation. Count how many gates the signal made it down the line before it stopped, multiply by the propagation time of each gate and Bob’s your Uncle.

Alternatively, the PIC24F series micros from Microchip have a CTMU module for this sort of thing. It’s basically a decent current source, a capacitor and an ADC, but it can theoretically interpolate between clock signals, turning a 16MHz clock into an effective clock speed of ~16GHz, or ~61 picoseconds per ADC level. Cool, huh!

Quite easy to measure the speed of light using a good old analog oscilloscope. No fancy arduino needed. Just a repetive pulsegen with short risetime (HP 8082A) and a fast CRO (Tek 7904A)

It could make for a useful rangefinder. Hmmm…

Easy with a longer distance. Did it in college physics down the hallway of the “natural science” building.

You need a bright flash, like a camera flash, 2 photocells and a mirror. Position 1 to see the flash, the other to see the reflection from the mirror way over there. Fire the flash and time the difference. Don’t forget to multiply the distance by 2.

For our stone age experiment in the mid 70′s, we used a strobe and oscilloscope. The physics department’s scope wouldn’t trigger, so I ran over and got one from electronics that I knew worked.

I believe it was once done with a lamp, a spinning wheel with vanes on it and a mirror in another village. He measured how much of the flash between the vanes he saw and given the rpm of his gizmo.

I love the chocolate method, and like all proper mad scientists, she ate her experiment when finished.

I will wait for someone compile all ideias and implement a cheap rangefinder with an ARDUINO, of course.