Disclosed are an optical system, in particular for microlithography, and a method for operating an optical system. According to one disclosed aspect, the optical system includes at least one mirror (100, 500, 600) having an optical effective surface (101, 501, 601) and a mirror substrate (110, 510, 610), wherein at least one cooling channel (115, 515, 615) in which a cooling fluid is configured to flow is arranged in the mirror substrate, for dissipating heat that is generated in the mirror substrate due to absorption of electromagnetic radiation incident from a light source on the optical effective surface, and a unit (135, 535, 635) to adjust the temperature and/or the flow rate of the cooling fluid either dependent on a measured quantity that characterizes the thermal load in the mirror substrate or dependent on an estimated/expected thermal load in the mirror substrate for a given power of the light source.