A lighthouse is a coastal navigation aid that helps ships and submarines avoid hazards and find safe routes at night or in poor visibility. Its bright lamp is placed high above the shore so the light can travel far over the ocean. Mariners use the direction, color, and timing of the light to identify the lighthouse and understand nearby dangers.
This matters because rocks, reefs, shallow water, and harbor entrances can be difficult to see from the water.
Key Facts
- Distance to the horizon increases with height: d ≈ 3.57√h, where d is in kilometers and h is in meters.
- A lens bends and focuses light by refraction, changing the direction of light as it enters and leaves glass.
- Fresnel lenses use stepped rings to focus light like a large lens while using much less material.
- Light intensity decreases with distance according to I = P/(4πr^2) for light spreading equally in all directions.
- A rotating lighthouse beam creates flashes because the focused beam sweeps past an observer at regular intervals.
- A lighthouse characteristic is its coded pattern of flashes, eclipses, colors, and period, such as 2 flashes every 10 s.
Vocabulary
- Lighthouse
- A lighthouse is a tower with a powerful light used to guide vessels and warn them about coastal hazards.
- Fresnel lens
- A Fresnel lens is a thin, stepped lens that concentrates light into a strong beam while reducing weight and thickness.
- Refraction
- Refraction is the bending of light when it passes from one material into another at a different speed.
- Characteristic
- A characteristic is the unique flash pattern, color, and timing that identifies a particular lighthouse.
- Navigation aid
- A navigation aid is any signal or marker that helps pilots or mariners determine position and avoid danger.
Common Mistakes to Avoid
- Assuming a brighter light always means a ship is safer is wrong because distance, weather, beam direction, and horizon limits also control visibility.
- Ignoring the flash pattern is wrong because two lighthouses can look similar, but their coded characteristics identify different locations.
- Treating a Fresnel lens as a simple magnifying glass is wrong because it is designed to focus lamp light into a concentrated beam for long range use.
- Forgetting Earth curvature is wrong because even a powerful light cannot be seen once it is hidden below the horizon.
Practice Questions
- 1 A lighthouse lamp is 36 m above sea level. Using d ≈ 3.57√h, estimate the distance to the horizon in kilometers.
- 2 A lighthouse has a characteristic of 3 flashes every 15 s. How many flashes would a sailor observe in 2 minutes if the pattern repeats continuously?
- 3 A ship and a surfaced submarine both see the same lighthouse, but one reports a white flash and the other reports a red flash. Explain how colored sectors can warn vessels about safe and dangerous areas.