Hiroshima shadows - how?

You know those pictures of the shadows of people etched into the walls in Hiroshima? So the Internet tells me that they happened because the bodies shielded the walls from the blast. But then shouldn’t the “shadows” be lighter in color than the walls? Why are they darker?

(For context - I’m creating lesson plans for Hiroshima this week, so I’d be grateful for an explanation that 7-8th graders could understand. :slight_smile: )

The people were incinerated in temps upward of 1000 C. What you’re seeing as a shadow is all that’s left of their body.

Sorry I don’t have an explanation, but I saw them, too. I remember some that were on a concrete stairway.I can still see the image in my mind some 30 years later.

Creepy!

Just to add to my previous post, in places where the impact was physically blocked by some object, the “shadow” is lighter, as you’d expect, as in this photo:

So the shadow is actually created by their ashes being seared into the concrete?

Here’s my educated guess:
The only place these shadows are found are on very strong, temperature-resistant materials - like stone or concrete. When the bomb exploded, anyone in front of one of these surfaces would be either incinerated, or badly burned (depending on how far away they were). Their body would act as a shield, preventing much of the radiation (Photons) from hitting the surface. The unshielded surface would absorb a full blast, and the top layer would become extremely hot, which would a) burn off any dirt or soot and b) cause the surface to ablate, exposing a fresh, lighter surface underneath. I think these two effects would tend to create dark shadows on a lighter background.

my recall is that the body caused a shadow which was blocking infrared light for whatever short time that light radiation and the body lasted. the unshielded noncombustible surface was affected by the heat.

Here’s a “shadow” on concrete steps of a bank building in Hiroshima, now preserved at the peace museum there. Sorta subtle, unlike the photo of bridge posts linked above.

Imgur

The shadow on the bank steps was originally much darker. It faded from exposure to the elements. BTW, it was only 250m from the hypocenter.

The first thing that hits you in a nuclear blast is a very powerful blast of hard gamma, x-rays, then ultra-violet, followed by visible, and then infra-red light. It all happens very fast. Much faster than a person can fall from the standing position. Although the person will in all likelihood be either burned to ash, or even vaporized by this, the spot immediately behind them is protected by the absorption of most of that energy. The energy released by the incinerating body is much smaller, and generally does not affect objects more than a few inches away.

The blast wave comes later. Perhaps seconds, perhaps a fraction of a second, depending on the distance from the weapon. (Difference between speed of sound in extremely hot air, and the speed of light, times the distance.) All trace of the remains may well have been destroyed by the blast wave, or the returning wave, or the firestorm which follows. (Everything is at or above kindling temperature, and the return blast is followed by much cooler oxygen bearing air from outside of the blast circumference.) The firestorm then rapidly expands outward as well.

So, a flammable object like a person, in front of a stone wall leaves a shadow. A literal shadow, in the “light” of the thermonuclear blast and is then obliterated itself by the secondary and tertiary effects. The shadow is, in fact, a shadow.

Tris

If you actually see the stairs in person, the shadows are much easier to see compared to what one sees in this photo.

So if I tell my students that the shadows are actually what remains of their physical body (ashes and soot, bascially), am I correct?

Sorry if I’m being dense. I read your explanation but I’m still not sure why the area being shielded is darker than the area that isn’t.

Imagine that light “bleaches” a surface.
Now imagine that a nuclear blast bleaches a surface but not where the light is blocked i.e. a shadow.

It’s similar to what might happen if you placed a couch by an open window. The part in front of the window would be bleached by the sun; the part against the wall would stay a darker color.

Oh, okay. Thanks, that makes sense. :slight_smile:

You’d get a Hell of a lot more heat from the visible, ultraviolet, and gamma parts of the spectrum. There’s nothing special about infrared being “heat”; all light behaves the same way. The only reason we associate heat with infrared is that at the temperatures of most things we’re familiar with, the peak is in the infrared. For a hotter source like a bomb, though, the peak will be at higher frequencies.

For full effect, imagine that the couch is nonflammable and well secured. Move the sun onto your front lawn for a few seconds and then take it away.

No, that is not correct.

At school a long time ago I saw quite a few pictures of the post-blast Hiroshima and Nagasaki (the presentation was considered sufficiently graphic that parents had to sign a consent for a kid to see it)

As I recall, some of the shadows were light, some were dark.

For example, if there was a white/light grey concrete wall painted a darker color and someone was standing in front of it, the blast might burn away the darker paint except behind the person, leaving a dark shadow.

Conversely, if the blast charred a surface, except where a person was standing in front of it, then the shadow might be lighter in color.

The shadows are where a person or object protected something else from the full force of the atomic blast, that’s all. It the difference between the blasted surface and the somewhat protected surface that makes the image.

Any ashes or soot generated, however, would have been blown from the area by the massive air movements caused by the blast, ditto for ay survivig body parts. The shadows are images, they aren’t actual human remains in the sense of being bits of burned body.

this is correct what broomstick wrote, actually took the words right out of my mouth…

If you simply had to be two years late, could you have at least contributed something to the discussion for fuck’s sake?