Two-Way mirrors allow 30% of light to pass through them, while the remaining 70% of light reflects from the bright side and cause the light passing through to be nearly indistinguishable. This gives off the appearance of a normal mirror on the bright side, but a window on the dark side, which allows people to see what is occurring on the other side of the mirror. This can be bad for privacy concerns and security purposes if not used in the proper settings.