|MLA Citation:||Bloomfield, Louis A. "Question 1193"|
How Everything Works 19 Jan 2018. 19 Jan 2018 <http://howeverythingworks.org/print1.php?QNum=1193>.
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.