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