I did an experiment this morning.  I usually have my first-year students drop a piece of plastic called a “picket fence” through a photogate to measure the acceleration of gravity.  The “picket fence” is a piece of clear acrylic plastic with some evenly-spaced black stripes on it.  The photogate is a black plastic bracket in a rectangular C-shape with an infrared LED on one part of the “C” and an infrared detector on the opposite side of the “C” so that the detector always “sees” the LED unless there is some object blocking the path.  There is a similar device in your TV remote control and TV set.  The LED is in the remote control and the detector is in your TV.  If something opaque is between the remote control and the TV, the TV won’t respond to the commands from the remote.

Anyway, for a number of years I have had difficulty with this experiment.  For some kids, it would not work, and for some kids it would.  I tried restarting the software that reads the photogate, switching picket fences, switching computers, switching photogates.   It seemed like the problems were random.  Yesterday, I tried out the experiment to make sure it would work.  No problem.  I tried to demonstrate the procedure this morning to my first class of the day.  It failed.  ACK!

I sent the kids on a break (we had two periods of class in a row, so this was usual) and switched out photogates again.  It worked!  Our oldest photogates are the ones that work!  I quickly found all the oldest photogates (just enough for the number of groups I have) and my students did the lab successfully.  Meanwhile, I did my own experiment.

I discovered that the newer photogates can’t “see” through my picket fences, in the clear spots.  Not when  I put the picket fence (clear area) next to the LED, not when I put it next to the sensor, and not when I held the photogate in the middle and slowly rotated it to see if there was a better angle for transmission.  Admittedly, I made these picket fences myself, and they are not perfect.  But why this problem?  Is it the type of plastic?  Is the plastic too thick?  Do the LEDs in the newer photogates emit a different wavelength of infrared light than the older ones?  Or is the beam less intense?  My next investigation will involve my digital camera (digital cameras can detect infrared – try looking in the LCD display while pointing the camera at your remote control and hit some of the remote control buttons.  If the remote control is functional, you should see the flashes of the infrared LED).

So a weird thing is, I have a small TV in my office, and sometimes it turns itself on when I turn on the overhead light, which is a fluorescent fixture.  Why does my TV respond to the light turning on?  It only happens sometimes – maybe once a month.  I turn on the light in the office most days, so it is only 1/20 to 1/30 of the times I turn on the light.


4 Responses to “Infrared”

  1. Celia Says:


  2. Greg Says:

    Or a meddling spouse trying to go all Gaslight on you.

  3. Rhett Says:

    It is probably ghosts.

    No really, for the IR thing, I would look through your home made fences with a video camera. The video camera can see the IR light and you can tell if it is passing through the clear parts or not.

  4. teawithbuzz Says:

    @Greg: except if a meddling spouse WERE going all Gaslight then he wouldn’t suggest it. Buzz is more likely than you, anyway – you can’t wait that long for a laugh.

    @Rhett: that’s the plan….maybe next week after Physics Olympics is over and I finish grading the tests I’m giving tomorrow…and when I am more FULLY recovered from this darn upper respiratory crapola.

    Actually, I posed the question to the CEO of the company that manufactured the photogates, and he suggested that the picket fences manufactured by his company would work with no problems. Then he offered to send me a couple. Now THAT’s customer service!

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