A planar mirror is a type of reflector that can be used in an optical microscope to direct light. When the illumination beam is directed through the optical path and onto the specimen, the mirror serves as a sole reflector, reflecting all of the incoming light back towards the eyepiece or image sensor. However, when the projection electron microscope is in use, an X-ray source generates an x-ray beam which passes through the specimen without being reflected back by the mirror. This allows for superior resolution due to the lack of diffraction artefacts created by reflection from soft tissues and other obstacles in between the X-ray source and detector