Science Puzzle
How Can a Lighthouse Be Seen So Far Away?
Many lighthouses have used a single electric bulb of about 1,000 watts, not hugely brighter than a powerful floodlight. Yet the beam can be seen by ships 30 or 40 kilometres out at sea. A bare bulb like that would fade into the dark long before that distance.
How can a lighthouse be seen so far away?
The Answer
A huge lens round the lamp gathers light that would otherwise spread out in every direction and bends it into a narrow beam. All that light is concentrated where the ships are, so it stays bright for tens of kilometres.
A bare bulb sends its light out in all directions: up into the sky, down into the sea and everywhere in between. Only a tiny fraction goes towards any one ship, and it spreads thinner and thinner with distance. A lens bends the light so the rays come out almost parallel, like a magnifying glass working backwards. Squeezed into a narrow beam, the light in that direction can be many thousands of times brighter than from the bare bulb.
A single glass lens big enough would be enormously thick and heavy. In 1822 the French physicist Augustin Fresnel had a better idea: cut away the useless middle of the lens and keep only its curved surfaces, as a set of thin, stepped rings. His first lens was installed in the Cordouan lighthouse in France in 1823, and within a few decades lighthouses all over the world had them. Many lenses turn, so the beam sweeps round as a flash, and each lighthouse has its own pattern of flashes so sailors can tell which one they're looking at.
The principle: Focusing light into a beam. A lens that bends a lamp's light into a narrow, nearly parallel beam concentrates it in one direction, so it stays bright over long distances.