Einstein, in his 1905 relativity paper Electrodynamics of Moving Bodies, allocates a section to the issue of reflection of light from a moving mirror and derives three formulas (angle, frequency, energy). What was his reason or motivation behind including this particular problem?
Near the bottom of Page 18 of above reference Einstein, in relation to his formula for pressure of light on a moving mirror, says "in agreement with experiment and with other theories". What experiment and theories is he referring to?
Was there any related experiment at that time that showed discrepancy with the classical formulations? For example were there any observed aberration that indicated an issue?