When the laser beam passes a magnetic field or reflects on a magnetized plane, the plane of polarization of the reflected beam slightly rotates due to the magnetic field. Using this magnet optical phenomenon called the "Kerr effect", recorded data on the MO disk is read.

As shown in the figure, plane of polarization of the reflected laser beam striking the recording layer slightly rotates according to the magnetic direction of the point.

This Kerr rotation angle is converted into a change of light intensity using an analyzer (Polarized Beam Splitter).
And this is detected by the photodetector to be converted into electric signal for reading data.

The intensity of a laser beam at reading is set under one seventh of writing and erasing level and the recording layer will not be disturbed.