A chart writes that name in a few letters and numbers: Fl(2) W 10s means two white flashes, repeated every ten seconds. Type any characteristic below and watch it.
Try
Fl(1+4+3) W 45s0 s45 sWhite flashes in groups of one, four and three, eight in all, the whole sequence repeating every 45 seconds.
Time it first. Count it second. Read the letters last.
Most guides start with the letters. This one thinks that is backwards. At night you see a point of light that comes and goes, and the surest thing you can measure is how long it takes to return to where it started: start a count at the first flash of a group, stop at the first flash of the next. That number is the period, and charts print it last, as 10s. Then count the flashes in a group. Only then match what you saw to the letters.
The test is below. Hatteras, Portland Head and Fastnet all read Fl W: a single white flash. Only their periods, 7.5, 4 and 5 seconds, tell them apart.
Eight real lights
Characteristics as listed in each lighthouse's Wikipedia article; heights and ranges in metres and nautical miles.
Minot's Ledge
Off Cohasset, Massachusetts, USA
Fl(1+4+3) W 45s 26m 10M0 s45 sWhite flashes in groups of one, four and three, eight in all, the whole sequence repeating every 45 seconds. The light stands 26 m above high water. Its charted range is 10 nautical miles.
One flash, four, then three. Since 1 May 1894 the light has flashed this group, and because "I love you" has one, four and three letters, the tower is known locally as the "I love you" light. Count the dark between groups: it is what separates 1, 4 and 3 from a run of eight.
Fl W 7.5s 57m 24M0 s7.5 sOne white flash every 7.5 seconds. The light stands 57 m above high water. Its charted range is 24 nautical miles.
A one-second flash and a six-and-a-half-second eclipse: 7.5 s in all. The flash count tells you nothing here, so the period has to: time two flashes, not one.
Fl(2) W 15s 44m 20M0 s15 sTwo white flashes close together, then darkness; the group repeats every 15 seconds. The light stands 44 m above high water. Its charted range is 20 nautical miles.
Two flashes close together, then a long dark. A group is read as one word: the pair belongs together because the gap inside it is much shorter than the gap after it.
Main lightFl(2) W 10s 41m 17M0 s10 sTwo white flashes close together, then darkness; the group repeats every 10 seconds. The light stands 41 m above high water. Its charted range is 17 nautical miles.
Second lightIso R 10s 28m0 s10 sRed light and darkness in equal halves, one full cycle every 10 seconds. The light stands 28 m above high water.
Two lights on one tower. The main light flashes white in pairs; a lower window shows a red isophase light toward the Hand Deeps, a hazard to the west-northwest. See the red and you are in the arc that covers the danger.
Main lightFl W 15s 118m 27M0 s15 sOne white flash every 15 seconds. The light stands 118 m above high water. Its charted range is 27 nautical miles.
Second lightF R 111m 8MA fixed red light: steady, never dark. The light stands 111 m above high water. Its charted range is 8 nautical miles.
A slow white flash high on the headland, and a fixed red light seven metres below it with a far shorter charted range: 27 nautical miles for the white, 8 for the red.
F W 27m 19MA fixed white light: steady, never dark. The light stands 27 m above high water. Its charted range is 19 nautical miles.
A fixed light: no rhythm at all. A steady light says nothing distinctive about itself, which is why the IALA buoyage system does not use fixed lights on its marks.