During the freezing rain, beautiful rows of icicles formed on the cornice of a neighboring house. The icicles were separated very evenly and looked like a comb. I took pictures of them, and accidentally saw a very impressive phenomenon that has a simple physical explanation. The photo on the left was taken during the day, and the icicles were white as ice is supposed to be. The right photo was taken in the dark with a flash. It would seem that the icicles should sparkle even brighter, but they are black! Here's an explanation based on a simple law of optics: the angle of incident light is equal to the angle of reflected light. My camera in both cases was located about one meter below the icicles. Therefore, the beam of the light from the flash was coming to the icicle from below, and when reflected from it, went up and did not fall into the camera lens. This is why the icicles in the right photo are not well illuminated and look black.

Nikon Coolpix P900

Cropped, combined