resurgence2012
New Member
Hi Everyone,
I am new to this forum.
I am teaching high school science and I have been trying to find a simple, accurate and cost effective way to measure th speed of light.
I know that it can be done easily with a 100 MHz oscilloscope, pulsed laser and photodiode. However, my school does not have these resources, so I have been trying to find an easier, and cheaper way.
I considered trying to recreate the experiment done by Fizeau, where he used a rotating cog wheel, as a beam chopper and shone a light some 15 km before reflecting it back at the same cog wheel. I like this apparent simplicity of this approach, and it is also fairly easy for younger students to understand.
However, this method still seems to present a number of difficulties: manufacturing the disc, obtaining a fast enough motor, setting and aligning the optics, etc.
So more recently I was considering building a circuit that could be used to pulse a laser at say 20KHz, splitting the beam, and after send one half of the beam 300 meters, using a phase detector circuit like an XOR gate to detect the phase difference of the pulses, and using that, together with a knolwedge of their frequency, to calculate the time of flight. Of course this defeats my aim of having younger students or ideed 99% of my older students understand what's happening.
Notwithstanding, given the suggested frequencies etc, and a time delay of about one micro second, could I use a CD4046 CMOS? If so could anyone point me in the direction of a reliable easy to follow circuit diagram?
Alternatively, does anyone have any other suggestions for doing this experiment, perhaps more easily, but more accurately than just sticking a bar of chocolate in a microwave oven and measuring the distance between the hot spots?
Then again how difficult is it to build a stop watch that can literally measure times as short as a few nano seconds. Is there a way of doing this 'crudely' with a simple high speed RC circuit a couple of photodiodes and a very fast flip flop. Charge up the capacitor, send a sinlge pulse from a laser that first goes to a beam solitude. First half of singalong goes to flip flop which triggers the discharge of your capacitor through its associate resistor. The second half of the laser's pulse goes the length and back again of the school's playing field and hit the second photodiode which switches the capacitor off from further discharge and simulataneously contact to something that can measure the remaining voltage across the capacitor as indication of the te
Of flight. This feels like it could be the simplest and most elegant solution, but really I have no idea if it can be made to work
Thank you very much for your patience in reading this and any thoughts or ideas you share.
Cheers Peter
I am new to this forum.
I am teaching high school science and I have been trying to find a simple, accurate and cost effective way to measure th speed of light.
I know that it can be done easily with a 100 MHz oscilloscope, pulsed laser and photodiode. However, my school does not have these resources, so I have been trying to find an easier, and cheaper way.
I considered trying to recreate the experiment done by Fizeau, where he used a rotating cog wheel, as a beam chopper and shone a light some 15 km before reflecting it back at the same cog wheel. I like this apparent simplicity of this approach, and it is also fairly easy for younger students to understand.
However, this method still seems to present a number of difficulties: manufacturing the disc, obtaining a fast enough motor, setting and aligning the optics, etc.
So more recently I was considering building a circuit that could be used to pulse a laser at say 20KHz, splitting the beam, and after send one half of the beam 300 meters, using a phase detector circuit like an XOR gate to detect the phase difference of the pulses, and using that, together with a knolwedge of their frequency, to calculate the time of flight. Of course this defeats my aim of having younger students or ideed 99% of my older students understand what's happening.
Notwithstanding, given the suggested frequencies etc, and a time delay of about one micro second, could I use a CD4046 CMOS? If so could anyone point me in the direction of a reliable easy to follow circuit diagram?
Alternatively, does anyone have any other suggestions for doing this experiment, perhaps more easily, but more accurately than just sticking a bar of chocolate in a microwave oven and measuring the distance between the hot spots?
Then again how difficult is it to build a stop watch that can literally measure times as short as a few nano seconds. Is there a way of doing this 'crudely' with a simple high speed RC circuit a couple of photodiodes and a very fast flip flop. Charge up the capacitor, send a sinlge pulse from a laser that first goes to a beam solitude. First half of singalong goes to flip flop which triggers the discharge of your capacitor through its associate resistor. The second half of the laser's pulse goes the length and back again of the school's playing field and hit the second photodiode which switches the capacitor off from further discharge and simulataneously contact to something that can measure the remaining voltage across the capacitor as indication of the te
Of flight. This feels like it could be the simplest and most elegant solution, but really I have no idea if it can be made to work
Thank you very much for your patience in reading this and any thoughts or ideas you share.
Cheers Peter
