What's the electromagnetic effect on a pendulum oscillation?

[TorNaGul]

Hello,

This is just a curiosity of mine, after noticing light falling on a metal pendulum while doing a simple pendulum period measure, in the laboratory, using a mathematical pendulum at a small angle. Given that, there is a (very small) effect on the pendulum caused by the light falling on an area of the sphere, how can one include the effect of the light falling on the particular area, in regard to it affecting the period of one complete oscillation?

 

[mfb]

Light pressure? If the light intensity does not depend on the position of the pendulum, it won't have an effect at all.
For the general case: find the force everywhere, use perturbation theory.

Note that other effects like the motion of air in the room are orders of magnitude stronger.