Amplitude - intensity of oscillations, or in other words, the difference between the peaks and troughs. Oscillating timers are free running - in other words they don't have to be restarted at each period. And they will continue under constant conditions even though the response will eventually damp out. This is analogous to a spring-loaded watch. This is called entrainment and the signal that synchronizes the rhythm is the zeitgeber.
Light is the zeitgeber.
Both blue and red light are important. Transducing mechanism for photoperiodism A. Photoperiodism as an hourglass timer. Similarly, the flowering timer would be reset during the day when Pfr levels are re-established. B. Photoperiodism as an oscillating timer. A more recent idea, and one that has greater support, is that flowering is controlled by an oscillating timer.