磁光效应是电磁波在被施加准静态磁场物体中传播的种种现象。在这些旋磁材料中，左旋和右旋椭圆偏振光可以以不同速率在介质中传播，导致一些很重要的效应。当光线经过一层磁光物质后，会导致法拉第效应：光线的偏振面可以被旋转，成为法拉第旋光器。当光线被磁光物质反射后，会产生磁光克尔效应（不要与非线性克尔效应混淆）。

一般而言，磁光效应会造成局域性的时间反转对称破缺（也就是当只有考虑光线的传播而非磁场源的时候）以及劳伦兹互换性的破缺，这些现象是制作光学隔离器等设备的必要条件（光线在其中只往特定方向前进）。

二个偏振方向相反的旋光性物质，对应于无损介质的复共轭ε张量，被称作光学异构体。

A magneto-optic effect is any one of a number of phenomena in which an electromagnetic wave propagates through a medium that has been altered by the presence of a quasistatic magnetic field. In such a material, which is also called gyrotropic or gyromagnetic, left- and right-rotating elliptical polarizations can propagate at different speeds, leading to a number of important phenomena. When light is transmitted through a layer of magneto-optic material, the result is called the Faraday effect: the plane of polarization can be rotated, forming a Faraday rotator. The results of reflection from a magneto-optic material are known as the magneto-optic Kerr effect (not to be confused with the nonlinear Kerr effect).