Step-by-step explanation:
The explanation for this is that the space outside is illuminated with light from the sun. The person inside has either a lesser or no illumination at all. The dark inside room appears dark, and darkens the surface of the window facing inside, making it act like the silvered surface of a mirror. When the outside observer looks at the window, the window acts as a mirror, reflecting the light from the sun onto the eyes of the outside observer. For the inside observer, some of the illumination from outside finds its way inside, since the outside window surface is well lit, carrying with it the sensation of vision onto the eyes of the inside observer.