The transmission of light through single crystals of EuO and EuS has been measured as a function of magnetic field, temperature, and wavelength. Below the Curie temperature, the intensity of light reaching the photodetector abruptly diminishes to nearly zero as the applied field is decreased below a critical fieldHcrit. This effect is caused by diffraction of light by magnetic domains. Photographs of the spatial distribution of transmitted light forH<Hcritshow that the light is rather uniformly distributed over large angles. This diffraction effect is largest near the optical band edge and diminishes at longer wavelengths. By measuring the temperature dependence ofHcritone may determine the ferromagnetic Curie temperature in a novel manner. For EuO we findTC=69.5°K, which agrees well with a Curie temperature determination on the same sample by a more conventional magnetization method.