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 Scitoys Message Board » I need help with my Science Fair project » Is there a way to measure the distance a laser pointer is pointed at (like an object 100 yards out) « Previous Next »

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scott
 Posted on Wednesday, September 29, 2004 - 8:13 pm:

same as above

Simon Quellen Field (Sfield)
 Posted on Wednesday, September 29, 2004 - 10:59 pm:

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

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
 Posted on Thursday, September 30, 2004 - 8:07 pm:

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
 Posted on Saturday, October 2, 2004 - 3:38 pm:

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)
 Posted on Saturday, October 2, 2004 - 4:55 pm:

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
 Posted on Saturday, October 9, 2004 - 4:08 pm:

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)
 Posted on Saturday, October 9, 2004 - 4:15 pm:

I am an amateur like you. ;-)
You could buy or borrow an oscilloscope.
Schools have them. You could use one there.

Andrew
 Posted on Sunday, October 10, 2004 - 12:42 pm:

How about something like this? (But I don't really understand it)

Simon Quellen Field (Sfield)
 Posted on Sunday, October 10, 2004 - 2:14 pm:

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
 Posted on Sunday, October 10, 2004 - 3:56 pm:

Ok, thanks. I guess I will just have to find an oscilloscope.