The pendulum’s natural motion is given by the laws of gravity (Newtonian gravity, in this case). But the silicon disc is an excellent mirror: it can be pushed around by light.

In free fall, the acceleration IS g, so one could write the vertical position as the function: so this g could be called the acceleration due to gravity. HEY WAIT!

There are various methods to measure acceleration due to gravity. At Higher level all rely on one of the equations of motion. The measurements required will depend on the equation used. Watch this ...