A few days ago I put my thumb on my phone's camera as a manual way of shutting off the feed, and I was surprised that instead of black, the screen showed pink: light entered through my finger and into the phone's camera! I tried shutting the camera with other objects, and lo and behold the camera was showing black with all of them. And now I read your post, about a very practical and life-saving way of using just that observation basically! Well done!