I walked the ruins of fires. I saw white papers saying "In Mourning" on the front doors here and there. It means someone of their family has passed away recently. This is a custom of Buddhism.
Heat rays of the Atomic Bomb burned only the letters, "In Mourning," written in the black ink on the white papers. White parts of the paper remained unburned.
As a child, I played using a sheet of paper and a lens. The sunlight converged through the lens burned the black parts of the paper. The same thing happened when the Atomic Bomb exploded.