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I was just responding to other people calling it a mirror but the fact i cant see the left hand cord behind the disc makes me think it might be. Even if it is glass i think you'll still get a harsh reflection on it.
You've got a room flooded with day light during the day so i'm not sure why you'd want to project that much light onto the ceiling at night (if it is glass).
EDIT: Maybe it could be useful for certain times during the day when you're not getting direct daylight but still not a fan for the evening.
Is there a reason everyone is so sure that it is a mirror?
On first look I assumed it was glass, and the reflection of that little bit of string is just reflection you'd get with a normalish glass (there is nothing else, like the sofa showing). The angle of normal glass would give a bit of directional light reflection as well as refracting the light upwards - which based on the bulb is what I assumed the main purpose is.