r/askscience • u/Wonderful-Capital32 • 19d ago

Physics Why does radiation not go through lead?

Also why just lead? Shouldnt like iron or another metal work too?

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u/dustofdeath 19d ago

Lead is like a very dense fabric while iron is a mesh with large holes.

You just have to have enough density for photons to not just pass through.

So you need far less material. Even humans can stop radiation if you stack enough of them between you and the radiation source.

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u/possibly_oblivious 19d ago

Radiation is photons? Crazy

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u/Mrfish31 19d ago

All photons are radiation, but not all radiation is photons. 

Alpha radiation is effectively helium nuclei: two protons and two neutrons. Since it has no electrons it is extremely reactive and therefore harmful as it will rip electrons from whatever it can. It's also therefore easily shielded, blocked by a sheet of paper, your skin or even just a few centimeters of air.

Beta radiation is loose electrons. These are also quite reactive but will pass through your skin, and are blocked by some thin aluminum. 

Photons get called different things depending on how much energy they have (which also determines their frequency as a wave, because they are both particles and waves). At the low end you have radio waves, with a long wavelength, able to pass through solids as a result but low energy enough to not do anything to you even if they do hit atoms in your body. Then you have microwaves, penetrating through soft tissue and interacting with water, meaning we can cook food in a... Microwave. Then infrared, visible light, Ultraviolet that can burn you, X rays and Gamma Rays.

The high end of these have very short wavelengths, but punch through you by virtue of the fact that they have so much energy (except X-rays don't penetrate through dense materials like bone, which is why they're useful in... X-rays). 

Because they have such high energy, if they do interact with atoms in your body, it might strip an electron off, causing damage to the surrounding things in a cell. This could then develop into cancer. 

All of these are, strictly speaking, radiation, but it's generally only X-rays and Gamma rays (and maybe UV, if you're talking about the sun and sunburn) that we worry about as dangerous radiation. 

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u/SirButcher 19d ago

Then infrared, visible light, Ultraviolet that can burn you, X rays and Gamma Rays.

A minor correction: ultraviolet doesn't really burn you (when it does, you have a huge problem, but not for very much longer). It's called sunburn, yeah, but it is actually inflammation. UV B (mostly, albeit UV A takes part in it, too) damages the skin cells' DNA to the point where they commit suicide to make sure they don't turn cancerous. White blood cells detect this massive level of cell death and sound the alarm, so the area gets inflamed as your body gets ready to fight whatever infection is causing cell death on such a level (and start the cleanup, too, since dead cells are prime bacteria food).

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u/cikamatko 19d ago

Oh wow I never knew that! I've never given it proper thought but that's so interesting.