V3 draft / A

Optics · teaching model

How a lighthouse shines

The lamp in a lighthouse with a rotating Fresnel lens does not blink. It shines all the time, and the flash is seen by whoever the beam from the rotating optics happens to point at.

Diagram from above: the lamp, rotating lenses and the observer The lamp stands in the centre. Four numbered lens panels rotate around it; a beam 24 degrees wide comes out of each. A stationary observer stands on the right. In the initial state the beam of panel 1 hits the observer. 1 2 3 4 lampalways lit lensesrotate observer Timeline of one rotation: 12 s, flashes at 0, 3, 6 and 9 s 0 s3 s6 s9 s12 s
Cycle record: flash centres at 0, 3, 6, 9 s; each flash lasts 0.8 s and the gap between flashes is 2.2 s. The head turns once in 12 s, but the signal repeats every 3 s.

Rotation step by step

The controls require JavaScript. Without it you see the initial state of the drawing, and the whole cycle is described by the four frames below.

time
0.0 s
head
0°

observer: sees a flash

Step 1 of 16

Start: the beam of panel 1 hits the observer. Press “Next” to see the following events of the rotation.

Number of lens panels

One step is the next event: the start of a flash, its centre, its end or the middle of the dark gap.

Lamp
Stays in place and shines without a break.
Lenses
Numbered panels rotate around the lamp.
Beam
Width 24°. All of them shine all the time; the one that hits the observer is shown solid.
Observer
Stays in place, in the 0° direction (the right-hand side of the drawing).

Version without motion · model for 4 panels

The same cycle, four frames

One rotation takes 12 s. Every 3 s the head has turned through a further 90° and the next panel hits the observer. The signal therefore repeats every 3 s, although a full rotation takes 12 s.

Interval between flashes: Δt = T ÷ n = 12 s ÷ 4 = 3 s. Flash duration: 24° ÷ 360° × 12 s = 0.8 s.

  • 1234
    0 shead 0° · panel 1 hits
  • 2341
    3 shead 90° (¼ turn) · panel 2 hits
  • 3412
    6 shead 180° (½ turn) · panel 3 hits
  • 4123
    9 shead 270° · panel 4 hits

After 12 s the head returns to its starting position and panel 1 hits again. The rhythm is set by the number of panels, not by the length of a full rotation.

Comparing two models

A similar flash, two different devices

On the left the flash occurs because the optics rotate. On the right the optics stand still, and a controller switches the source on and off. Both drawings show the same moment as the main drawing.

Model of a Fresnel lens: the lamp in the centre, panels rotating around it, beams sweeping across the observer Fresnel (model) the optics rotate
Fresnel lens (model)The lamp shines without a break, and the optics rotate. It has a rotation period T and — with n equal panels — a signal period T ÷ n.
Model of an LED flasher: a stationary projector, a controller in the middle, the beam directed at the observer controller LED flasher (model) nothing rotates
LED flasher (model)The projector stands still, and the rhythm is set by the controller’s programme. There is no rotation period here; the pattern was set to imitate the chosen lens.

These are two separate models, not two modes of one device. That the observer sees a similar rhythm results from how the model is set up, not from the two techniques being identical.

Historical context

Rozewie: background, not proof

The dates alongside were copied from the working version V2, which cites source [1]. In this draft they have not been checked against the source. They do not imply that the drawing above reproduces the optics at Rozewie or its present equipment.

No photograph

What is needed is a legally usable photograph of a Fresnel lens of the relevant period, with a documented licence. Until then we insert no image: the drawing above is our own diagram, not a document.

  1. 1822

    Completion of the older lighthouse; initially oil lamps. [1]

    to be verified
  2. 1866

    First-order Fresnel lens. [1]

    to be verified
  3. 1875

    A second lighthouse alongside; two fixed lights. [1]

    to be verified
  4. 1910

    Raising of the older tower, Fresnel optics and a 1000 W bulb. [1]

    to be verified

Sources for further verification

Limits of the model

  • T = 12 s and a beam width of 24° are assumptions of the drawing, not data for any lighthouse.
  • The panels (1–4) are equal and evenly spaced; the beam is a uniform sector, without intensity distribution, vertical divergence, distance or visibility threshold.
  • Flash duration = 24° ÷ 360° × T = 0.8 s; gap = Δt − 0.8 s.
  • We do not show real light characteristics, ranges or hours. The page is not a navigation aid.
  • Flashes last 0.8 s and occur no more often than every 3 s; nothing flashes across the whole screen.