The Second Reflection!
Transcript:
Have you watched a crescent moon? A moon that has a tiny sliver of lighted surface. If you watch closely, that crescent moon, you can see the full circle also. You can see the part of the moon that does not get direct sunlight. And obviously you are watching the moon crescent, which means you are on the night part of the day—which means you are not getting direct sunlight either. So how is it possible that you are able to see the full circle of the moon? So the way it works out is the part of the moon that is lit, the crescent part, reflects that light to earth, where you are seeing the crescent part of it. And then the earth reflects that light back to the moon. And that reverse, or the second reflection, is what lights up the moon enough that you can see the unlit part of the moon also. Pretty interesting. Same thing happens with an iPhone. If you are in a dark room and you want to do face recognition, the light from the screen is enough to light up your face and reflect it back to the camera, so that the face recognition happens. It's the second reflection which is the key.