in ww2 when America dropped the atomic bombs on Japan, there are images after of shadows of people being left on the street. What is the science behind that?
The first wave of energy that reaches somewhere after a large explosion is the light. Just like for example the sun it heats up everything in its path. But unlike the sun the light from a nuclear explosion is powerful enough to scorch concrete and set fire to paint. But just like the light from the sun it does not go through flesh and will cast shadows. What you are seeing is the unburned walls in the shadow of people.
There is marvel comic movie of Iron man , that shows when some type of experimental atomic explosion /heat – battle between good and evil takes place , the shadow human imprints were left on the wall and ground.
The explosion from the atomic bombs was so powerful and produced so much heat that it basically bleached and scorched every surface in direct exposure to it. The bodies of people acted as enough of a shield to block some that heat and radiation, leaving a permanent shadow.