The glass in the rear view mirror is cut so that it forms a thin wedge--it's thicker at the top than it is at the bottom. Its back surface is fully mirrored by a layer of aluminum. For daytime use, the mirror is oriented so that light from behind the car enters the glass, reflects from the layer of aluminum on the back surface, and returns through the glass to your eyes.
But when you tip the mirror upward for night use, the mirrored back surface presents you only with a view of the car's darkened ceiling. However, there is a weak second reflection from the clear front surface of the mirror--whenever light changes speeds, as it does upon entering the glass, some of that light reflects. About 4% of the light striking the front surface of the mirror from behind the car reflects without entering the glass and is directed toward your eyes. Since the image you see is about 25 times dimmer than normal, it doesn't blind you the way a reflection from the mirrored surface would.
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