Why hanging a mirror at the door can ward off evil spirits?

In geomantic omen, don't hang a mirror in a normal porch, because the reflection of the mirror will affect the family's wealth, except for the following situations: the porch faces the south (only palaces or royal buildings can curb the south), the porch faces the road (turbulence disturbs households), and there are unclean or ominous things (garbage stations, directional arrows, etc.). ) towards or near the door, and the entrance is facing the sharp corner of other buildings (rushing)