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Why do Searchlights Shorten ?

  1. Sep 29, 2004 #1
    Why do Searchlights "Shorten"?

    A few days ago I had the opportunity to stand near the base of a revolving searchlight (apparently part of an ad campaign for a car dealership). It was a clear night. An odd optical illusion struck me: the beam seemed to go out only a few hundred feet... and then stop completely in a short distance, as if stopped suddenly by a cloud or fog bank. The operator told me that the beam actually reached 5 miles and I biked around and verified that it looked that way from the side. But still, from the base the appearance was very different. What could have caused this? My guess is it has something to do with a slight flaring of the beam which might mess with the perspective. Still it is an eerie and striking effect and if you ever have a chance to see a revolving searchlight from the base, don't miss it.
  2. jcsd
  3. Sep 29, 2004 #2


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    You only see the light that is reflected back to your position as observer. When you are near the light source, you are looking more or less parallel to the beam and the distant reflections are drowned out by the brighter, nearby reflections. When you move off at an angle and to a distance, the more distant reflections are no longer obscured.
  4. Sep 30, 2004 #3

    I don't think that could explain the entire effect. The beam seemed solid and approximately uniform in brightness until the last part of it, where it really dropped off in brightness quickly. Viewed far away from the side hardly any reduction in brightness was perceptible except far in the distance. I think the flaring of the beam altering perspective may have been the larger factor; however, viewed from the side very little flaring of the beam was visible.

    What type of equation would describe how quickly a searchlight's beam intensity drops off with distance? Would it be the same as for a regular light, or different because it is nearly straight instead of wedge-shaped?
  5. Sep 30, 2004 #4


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    Was the beam pointed up? It may have been a rapid change in atmospheric conditions.
  6. Sep 30, 2004 #5
    Just like the searchlight the officials use to call for Batman!
  7. Sep 30, 2004 #6

    No, the beam was mostly horizontal--maybe a 15-30 degree incline at a guess.

    You know, that Batman signal never made a lot of sense to me. What if the criminals did their dirty work on a night with no clouds?

    Something occurred to me--I wouldn't necessarily be able to see what flaring there was from the side, since the beam was rotating and I had no way of knowing just when it was exactly perpendicular to my line of sight. It would have appeared almost straight to me at SOME point in its rotation almost no matter what its flaring was. So the flaring really could have been significant.
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