Every light on a coast carries a signature: how many flashes, in what group, and how long between them. A mariner who cannot see the land can still identify the light by its rhythm, and the rhythm is published in a book — which means the coastline is, in a sense, a piece of music that ships can read.
The character of a light is written in a very compressed code. Fl(2) means a group of two flashes, W means the group repeats every twenty seconds, and everything else — the range, the colour of the light, the height of the focal plane — is in the same table. A mariner's light list is one of the densest pieces of information in the whole of navigation.
And there is nothing romantic about it. A lighthouse exists because a rock at night is invisible and a wreck is expensive, and the argument for building one was always made in pounds.
A first-order Fresnel lens does not make a thin sheet of light. It concentrates the light from a lamp into a narrow horizontal fan — bright in one plane, dim in the other — so that a mariner sees a flash when the beam sweeps across him and almost nothing the rest of the time.
The flash is the whole point. A steady light tells you there is a light; a flash tells you which light, and how far away, from the time between the flashes. Fog thickens the beam into a visible cone and shortens its reach at the same time, and the two effects are the reason fog signals were invented: sound carries where light does not.