Why did the atomic bomb dropped on Hiroshima leave shadows of people etched on sidewalks?

Christopher Nolan's Oscar-winning biopic Oppenheimer has reignited public interest in the historic race to develop atomic weapons.

Yet, while the film dwells on J. Robert Oppenheimer's uneasy conScience and his fear of a future nuclear war, being told from his perspective, it doesn't show the direct consequences of the Hiroshima and Nagasaki bombings. Yet on the streets of the two stricken cities, the horrifying evidence was plain to see. 

Black shadows of humans and objects, like bicycles, were found scattered across the sidewalks and buildings of Hiroshima and Nagasaki, two of the largest cities in Japan, in the wake of the atomic blast detonated over each city on Aug. 6 and 9, 1945, respectively. 

It's hard to fathom that these shadows likely encapsulated each person's last moments, similar to the ashen casts of ancient volcano victims preserved at Pompeii. But how did these shadows come to be? 

According to Dr. Michael Hartshorne, emeritus trustee of the National Museum of Nuclear Science and History in Albuquerque, New Mexico, and professor emeritus of radiology at the University of New Mexico School of Medicine, when each bomb exploded, the intense light and heat spread out from the point of implosion. Objects and people in its path shielded objects behind them by absorbing the light and energy. The surrounding light bleached the concrete or stone around the "shadow." 

In other words, those eerie shadows are actually how the sidewalk or building looked, more or less, before the nuclear blast. It's just that the rest of the surfaces were bleached, making the regularly colored area look like a dark shadow. 

Related: Why do nuclear bombs form mushroom clouds?