| Author | Message | ||
     scott |
same as above | ||
| Simon Quellen Field (Sfield) |
Yes. Point the laser at the object. Now move the laser one meter to the right. Now adjust the laser so it is pointing at the object again. Measure the angle you had to move the laser through to make it point at the same place as before. Now you know the base and two angles of a right triangle. A little trigonometry will get you the distance. In fact, if you have two lasers (or a beam splitter) you can mount the two lasers on a board a meter apart, one fixed, and one rotatable. A friend can walk ten feet away, and after you make both lasers point at him, you mark the amount of rotation of the movable laser as 10 feet. Then have your friend move to 20 feet away, make both lasers point at him, and mark that as 20 feet. Keep doing this, and you won't need trigonometry, you can just read off the distance on your homemade laser rangefinder. Another way is to use a 100 Mhz oscillator to turn the laser on and off every 10 nanoseconds. Use a telescope with a sensitive photodiode in the eyepiece to look at the spot. Feed both signals (the one turning the laser on and off, and the one from the photodiode) into an oscilloscope, and look at the phase shift of the two square waves. Each nanosecond is about 1 foot (30 centimeters). You could also feed both signals into an XOR gate chip, and get a pulse-width modulated signal out, reading 0 volts (after low pass filtering) for 0 feet, and 5 volts for 5 feet. Using a slower oscillator will get you longer ranges (10 Mhz will get you 5 volts at 50 feet). I'd go with the simple rangefinder method myself, although I have actually performed the second method with a 20 Mhz oscillator, and used it to measure the speed of light. | ||
| Andrew |
Measuring the speed of light with a laser pointer? Sounds like a science toy to me! Could you explain how to do this in more detail? | ||
| Andrew |
How would you set up such an expirement? What would the circuits look like? How would you calculate the speed of light? | ||
| Simon Quellen Field (Sfield) |
I have a digital oscilloscope that can collect 2.5 billion samples per second. This lets me measure events down to the nanosecond level. I power the laser pointer from a 20 Mhz oscillator, so the beam turns on for 25 nanoseconds, and then off for 25 nanoseconds, over and over. I connect the oscilloscope probe to the oscillator, so I can see the square wave on the screen. I connect another probe to a high speed detector usually used for fiber optic communications in stereo systems. I aim the laser at a mirror, so the beam bounces back to the nearby detector. Now I can see two square waves on the oscilloscope screen. I move the mirror back until the two square waves line up on the screen. Now I mark the spot where the mirror is. Then I move the mirror back some more, watching the bottom waveform on the oscilloscope move to the right, until it has moved so far that the two waveforms again line up. Since the waveform has moved by one full wave, I know that I have caused a 50 nanosecond delay, relative to where the mirror was at first. I measure the distance between the mark I made to record the mirror's first position, and the current position of the mirror. I multiply that distance by two because the laser went that far and then came back. Now I know that the light went 24 feet and 7 inches and back in one cycle of the 20 Mhz oscillator (50 nanoseconds). Multiplying by 20 million gives me 481,666,667 feet per second. Multiplying by two (for the return trip) and dividing by 5,280 feet in a mile, I get 182,449 miles per second. When I put the term "speed of light in miles per second" into Google, I am told it is 186,282.397 miles per second. So, I am about 2% away from the accepted value for light in a vacuum. But I am doing the experiment in air, and I am measuring to the nearest inch, and lining up waveforms on the oscilloscope by eye, so I am lucky to be that close. | ||
| Andrew |
Wow, thanks for such a detailed answer! I don't happen to have a digital oscilloscope, though. Do you know of any way of performing this experiment that is within reach of an amateur like me? | ||
| Simon Quellen Field (Sfield) |
I am an amateur like you. ;-) You could buy or borrow an oscilloscope. Schools have them. You could use one there. | ||
| Andrew |
How about something like this? (But I don't really understand it) | ||
| Simon Quellen Field (Sfield) |
I built a TV oscilloscope years ago, using a couple of cheap integrated circuits. It was fun for watching your voice, but it is too slow for measuring the speed of light. It was easier to build than the one in the link you posted, and did not require modifying the television at all. The TV oscilloscope paints about 500 lines per second. (It is "free running" so there is no need for a vertical retrace.) On each line, the signal waits an amount of time corresponding to the voltage level on the input, then it turns on for a microsecond to paint a dot on the screen. Thus the image is of a line wiggling around vertically in the middle of the screen. Unlike the link you posted, both halves of the signal are shown, not just the positive side. So, you can time things to 1/500th of a second with a TV oscilloscope, instead of the 1/2,500,000,000th of a second with the digital oscilloscope. Light travels about 600 kilometers in 1/500th of a second. So, to see the delay on a TV oscilloscope, you will need to move the laser back 600 kilometers. Unfortunately, the laser is only visible a kilometer or two away, and the horizon is less than 600 kilometers away, so the light would be blocked by the curvature of the earth anyway. The horizontal frequency of the television is about 30 times better than the vertical. This means that a delay of only 20 kilometers will show up. When a television signal is reflected by a jetliner, so your television gets one signal directly from the television tower, and another signal later from the reflection off of the jetliner, you can see the effect as "ghosting" on your TV. You see the main picture, but you also see a faint picture to the left of it, caused by the reflection. If you know where the TV tower is, and which direction the jetliner is, you can use the distance between the two images on the screen to determine how far away the jetliner is. Conversely, if you know where the jet is, you can calculate the speed of light, to within about one part in 15,000 (the resolution of the horizontal television signal). | ||
| Andrew |
Ok, thanks. I guess I will just have to find an oscilloscope. |
