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Nuclear bombs are harmful in proportion to their distance from you when they explode. I believe that, at short distances, the radiation (visible or not) from the bomb is so powerful and destructive that it burns things up right away before they're even hit by the shockwave. Is there a word for this destructive light?

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Yes. You just used it! radiation. You could call it electromagnetic radiation, if you want to distinguish high-energy photons from other kinds of radiation. But it's still radiation. – Peter Shor Jan 7 at 16:08
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@PeterShor I think there is a specific word for the short-range, destructive radiation from a nuclear blast though. It may be a compound word possibly containing the word 'flash'. However, I could be wrong. – sirdank Jan 7 at 16:13
    
I've heard it called the blast wave, thermal blast, blast energy, and nuclear radiation – anonymous Jan 7 at 16:20
    
Not disagreeing with the basic premise of the question (danger decreasing according to the square of the distance) but fallout is also carried on the wind. Also, when you bury irradiated tractors and trucks, and then dig them up some years later and drive them away, the radiation spreads. – Tim Romano Jan 7 at 16:36
    
The relative ranges of destruction from blast and thermal radiation are largely dependent on the size of the nuclear reaction. For very large weapons, thermal output can be lethal at ranges where blast effects may leave windows intact. See nuclearweaponsarchive.org for more information. – Eric Hauenstein Jan 7 at 21:57
up vote 25 down vote accepted

Nuclear weapons emit large amounts of thermal radiation as visible, infrared, and ultraviolet light, to which the atmosphere is largely transparent. This is known as "Flash". –Wikipedia

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+1 for thermal radiation – Graffito Jan 7 at 16:54
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Nuclear Flash sounds cool – Aequitas Jan 7 at 22:13
    
partly relevant link – E.P. Jan 8 at 13:54
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"Alright everybody it's showtime. Do not look at the flash. DO NOT look at the flash." – True Lies – Mazura Jan 8 at 21:05

It depends on what exactly you're talking about. The initial explosion creates a nuclear flash. It's this flash that blinds people miles away, who are far enough to survive the blast itself. In fact, blindness resulting from a nuclear explosion is called flash blindness. The most dangerous emission from within a nuclear flash, though, is called gamma rays. It's light in this spectrum that is most lethal to all life. Long after all of the searing heat of the nuclear flash has dissipated, gamma rays are still doing immense damage.

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Radiant heat from light travels at the speed of light, just like gamma rays. – Benjamin Harman Jan 7 at 18:08
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@BenjaminHarman 'Heat' itself doesn't travel at the speed of light. The gamma rays, however, do travel at the speed of light and can heat up whatever they hit (including air molecules.) Also, gamma radiation will be gone almost instantly. It's alpha particles that tend to stick around. The damage done by the gamma rays can (and likely will) persist, but the gamma rays from the blast themselves will be long gone before the blast pressure wave arrives. Radioactive fallout from the blast may continue to emit gamma particles that can cause damage much later, though. – reirab Jan 7 at 21:56
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@reirab : The heat from the sun's rays travels to earth with the light at the speed of light. It doesn't roll on in after after the fact. The heat from the sun comes from the light itself. The same is true from the intense light of a nuclear flash: There is a flash of heat that comes immediately upon witnesses. Now, the heat and the fire that the explosion creates at the blast site doesn't travel at the speed of light. It travels much more slowly. However, the heat that comes from the light of the flash itself is as instant as the flash, for the intensely bright light itself seers surfaces. – Benjamin Harman Jan 7 at 22:45
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This isn't theoretical. We've blown up nuclear bombs. People miles away have been instantly blinded and burned by the flash of light at the moment of detonation. I'm attaching a couple of sources to back this up. The Wikipedia article, next to where it discusses the initial flash, shows a picture of a man who was badly burned by the initial flash at Hiroshima. atomicarchive.com/Effects/effects2.shtml en.wikipedia.org/wiki/Effects_of_nuclear_explosions – Benjamin Harman Jan 7 at 23:00
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@BenjaminHarman I'm not suggesting it's theoretical. The problem seems to be a misunderstanding what actually constitutes 'heat.' To say that 'heat' itself travels at all is a bit awkward at best. Heat is a property of matter. No matter propagates away from a nuclear blast at the speed of light. If it did, it would have infinite energy, which it obviously doesn't. What happens with the sun and the Earth is the same as I described for with a nuclear blast. Electromagnetic radiation (gamma rays in this case) propagates away from the blast at the speed of light. – reirab Jan 7 at 23:07

This is a physics question. The atmosphere is opaque to radiation of charged particles and heavy particles. 'Luminance' or 'illumination' often implies just photons, and 'flash' implies visible photons.

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Nuclear weapons emit large amounts of thermal radiation as visible, infrared, and ultraviolet light, to which the atmosphere is largely transparent. This is known as "Flash". The chief hazards are burns and eye injuries. On clear days, these injuries can occur well beyond blast ranges, depending on weapon yield. en.wikipedia.org/wiki/Effects_of_nuclear_explosions – Benjamin Harman Jan 7 at 23:03

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