| 41. | Above the machine's exit, a green light signals all is well, a red light signals trouble.
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| 42. | It also provides a redundant stop light signal in the event of a stop lamp malfunction.
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| 43. | Messages for passengers could also be sent using a flag semaphore, and light signals at night.
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| 44. | When Robby realises that Erik knows about his collaboration, he fires a light signal and flees.
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| 45. | So both observers agree that the two light signals reach the observer in the train simultaneously.
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| 46. | Above the machine's exit, a green light signals all is well, a red light signals trouble.
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| 47. | That is because the light signals degrade and have to be amplified or reconstituted en route.
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| 48. | Laser range or other light signal frequency range finders operate just like radar at much higher frequencies.
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| 49. | There are not many standard light signals.
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| 50. | Aviation light signals are used in the case of a landing or navigation lights during night time.
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