The stepped lens Augustin-Jean Fresnel presented in 1822 replaced heavy silvered mirrors with concentric rings of prisms. It cut the mass of glass by an order of magnitude and still threw a brighter beam, because the prisms bend stray light back into the horizontal plane instead of letting it escape upward. A first-order lens stands over two metres tall and was, for a century, the largest precision optic anyone made.
Every lighthouse blinks its own rhythm, called its character, so a
navigator can identify the tower by timing the light alone — no chart
lookup, no bearing needed. Charts write it in shorthand:
Fl(3) W 20s means three white flashes in every twenty-second
cycle. Oc is occulting, where the light is on longer than it
is off; Iso splits the cycle evenly.
Most towers were automated through the second half of the twentieth century, and the last keepers left in the 1990s in most countries. A handful still keep a resident keeper, almost entirely for heritage reasons rather than navigational ones.
Satellite navigation made lighthouses redundant on paper decades ago. They stayed lit anyway, because a light needs no receiver, no power at the ship's end, and no working satellite — it fails visibly rather than silently, which is the property you actually want at three in the morning near a rock.