How Does a Lighthouse Beam Reach So Far?

Quick Answer

Lighthouses use a Fresnel lens, a set of concentric glass prisms that gathers light from a lamp and focuses it into a powerful narrow beam.

The Full Story

A bare flame or bulb spreads light in all directions, wasting most of it. In 1822, French physicist Augustin-Jean Fresnel designed a lens made from rings of prisms that bends light into a parallel beam, far thinner and lighter than a solid lens. The largest 'first-order' Fresnel lenses are taller than a person and can make a beam visible over 30 kilometres away. Rotating the lens creates a distinctive pattern of flashes so sailors can identify each lighthouse. Today many lighthouses are automated and use LED lamps.

Key Facts

1.The Cordouan lighthouse in France was the first to be fitted with a Fresnel lens, in 1823.

2.Early rotating lenses often floated on a bath of mercury to reduce friction.

3.Each lighthouse has its own 'light characteristic' printed on nautical charts.

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