USCG Exam PracticeLight List, ranges, and light characteristics
A mariner is comparing two lights: Light A is charted 'Oc G 4s' and Light B is charted 'Fl G 4s'. Both have a 4-second period. Which statement correctly distinguishes them?
- ✓ Light A is green for more than 2 seconds per period; Light B is green for less than 2 seconds per periodCorrect
- B. Light A is green for less than 2 seconds per period; Light B is green for more than 2 seconds per period
- C. Both lights are green for exactly 2 seconds per period because both have a 4-second period
- D. Light A alternates green and white; Light B produces a single green flash
Why A is correct
Occulting (Oc) has a duration of light longer than its duration of darkness, so in a 4-second period Light A is green for more than 2 seconds. Flashing (Fl) has a duration of light shorter than its duration of darkness, so Light B is green for less than 2 seconds per 4-second period. Equal durations would define Isophase. Alternating colour applies only to Al characteristics.