Light Speed: It’s not Just a Good Idea

[Kerry Wong] took apart a PM2L color analyzer (a piece of photography darkroom gear) and found a photomultiplier tube (PMT) inside. PMTs are excellent at detecting very small amounts of light, but they also have a very fast response time compared to other common detection methods. [Kerry] decided to use the tube to measure the speed of light.

There are several common methods to indirectly measure the speed of light by relating frequency to wavelength (for example, using microwave ovens and marshmallows). However, measuring it directly is difficult because of the scale involved. In only a microsecond, light travels almost 1000 feet (986 feet or 299.8 meters).

[Kerry’s] setup included a laser diode and an oscilloscope. He measured the time it took the laser to bounce off a mirror to develop a baseline time. Then he moved the mirror further away and repeated the measurement. By subtracting the baseline from the new measurement, constant delays in the test equipment cancel out.

The video below shows his results and also discusses more detail about the circuitry. We’ve seen people attempt this kind of measurement before with less success. If you’d rather walk before you run, you might think about measuring the speed of sound instead.

31 thoughts on “Light Speed: It’s not Just a Good Idea

    1. Ohh Bull Electrical, now you’re talking! As a lad I used to drool over all the weird things they advertised in the back of Practical Electronics, etc. They’re still trading, but the website is atrocious…..

  1. Use a laser and bounce it off the moon. That should make a short delay. They did it when the laser was brand new. They figure the tube detected a hand full of photons and it turned it into signal.

    1. You’d need a pretty hefty telescope on the end of the PMT to be able to detect a laser spot on the moon, wouldn’t you? The signal would be so weak among the sky scatter and actual moonshine that I’d imagine you’d need to send an encoded signal over the laser and use some radio-style signal processing to extract the laser signal from noise. A back of the envelope calculation tells me if you aimed a 5W laser at the moon, on the ground with a 6 inch telescope you’d receive about one or two photons a day (not counting any losses).

        1. You’re still looking at an extremely powerful laser to make it happen, and that’s assuming you can hit a retro reflector that’s at best a few meters wide. The only thing you have going for you is how incredibly spread out your laser will be by the time it gets there. If you want to do this, it would be far easier to go the way of the Hams and preform a moon bounce with radio waves.

          1. I thought that was one of the ways they measured distance with lasers by measuring beam spread. I assume the curvature of the moon’s surface and atmospheric scatter as it gets there makes it useless for us hobbyists.

    2. They (NASA, I think) still do this, pretty much every night as far as I know. The laser they use is a bit bigger than what we, as hobbyists, usually have access to, by several orders of magnitude.
      Also, they aren’t bouncing the laser off of the surface of the moon, they are bouncing it off of retro-reflectors placed there by the Apollo astronauts. The surface of the moon isn’t nearly reflective enough by itself.

      Something that we hobbyists can do is to bounce radio signals off of the moon. Amateur radio operators do it all of the time, it’s called EME (Earth-Moon-Earth) and it is handy for bouncing VHF signals over much longer distances than are normally possible (from North America to Europe, for example). I suspect that someone with an amateur radio license and the right type of gear could bounce a radio signal off the Moon and time the reflection.

  2. How long does it take for light to bounce BACK from the moon? How much distance does the earth move in that amount of time?

    Something strange about that light, isn’t there!

      1. The light takes 1.25 seconds to return to Earth.
        But let’s call it 1 second.

        In that time, the Earth has moves around the sun 30 km/s.

        Also in that time, the sun (and the Earth) have moved in our Galaxy 220km/s.

        Can the beam really bounce back? I don’t think so!

        1. Find something small on your desk. A battery, a bouncy ball, whatever.

          Now throw it gently straight up in the air.

          It landed in your hand (or on the floor), didn’t it? Not several kilometers away?

          Congrats, you just discovered the scientific theory, as well as the law of inertia.

  3. Actually, translation of the Earth should probably not be considered due to inertial frames of reference.
    But rotation should be considered. Rotation st the equator is 11km/s. Nearer the poles it could be closer to 6km/s. Which is the reported spread.
    But now I’m wondering how they measure to mm accuracy. Maybe they take the rotation into account. BUT they would never get the photon bounced back from where they sent it to the moon if the Earth rotates from underneath the beam. So how do they resolve to mm accuracy if there is a bran spread AND the Earth’s target from the bounce is not where they initially sent the beam?!!

  4. Laser rangefinder measurements of the distance to the Moon have become so commonplace and precise that in order to get the results they want, the researchers are having to take into account land tides — the entire continental plate may rise or fall several inches in the same way the sea rises or falls by several feet. Turns out everything in the Universe is pretty much made of rubber.

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