eli5 how does water stop radiation and not everything else outright, and how can radiation do damage to us if we are mostly made of water?
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Water is pretty decent at absorbing the light and particles radioactive things emit. This prevents that light and those particles from reaching something that’s alive and damaging it.
I don’t know what you mean by “and not everything else outright.” Water just has chemical properties that make it good at absorbing high energy light.
As for how radiation can still damage us if we’re mostly made of water, it’s simply because we’re not surrounded by an olympic sized swimming pool of water. You need a lot of water to meaningfully block radiation. The tiny amount of water on your skin or in your cells isn’t going to protect the proteins and other complicated molecules in them from getting hit by radiation.
Not only does it take a few meters of water depth to stop powerful radiation (more than even the fattest person), but our cells start on the outside. Radiation that only penetrates a little bit can still do a ton of damage.
As for how water stops harmful rays in the first place I’m not 100% sure but think it has to do with its material density. The rays hit the electron clouds of the water molecules and are blocked or deflected. The same happens with lead or any other suitably dense material.
In addition to what the other person has said:
>and how can radiation do damage to us if we are mostly made of water?
Because every little bit of us that isn’t water, which is still a lot, *really* does not appreciate getting hit by ionizing radiation.
Water is dense and a flying electron hitting lots of water loses its energy quickly.
It’s like shooting a gun at the tree. Tree is big and bullet stops when it loses all of its energy trying to pierce the wood. But when you shoot the plank made of the same wood it will get through.
Amount of water in us is like a thin plank, so radiation particle don’t lose much energy going through us.
Water is made up of molecules that are made of hydrogen and oxygen atoms. The hydrogen atoms have a positive charge and the oxygen atoms have a negative charge. This makes water molecules attracted to each other. When radiation comes into contact with water, the water molecules start to vibrate. This process continues until the radiation is gone. The water molecules absorb the radiation and then release it as heat.
So there are different types of radiation to begin with, the major groups would be neutron radiation, beta radiation (a free electron flying around), gamma radiation (also known as a photon or packet of pure energy, common example is sunlight), or alpha radiation (an alpha is a helium nucleus that is ejected with no electrons, so not an atom, just 2 neutrons and 2 protons stuck together flying around)
Two common ideals when discussing shielding are 1: what is the likelihood that radiation will interact with it and 2: if it does interact, how much energy is lost, and/or velocity change
To adress the first point density of the medium plays a big role, water is more dense than air, at a molecular level there is more “stuff” packed together for these particals to hit. However, the radiation varies too, the largest radiation particle is the alpha, this is the big rig truck of the radiation, it isn’t fitting through much, so typically a single sheet or paper or your dead skin layer is enough to stop this radiation, neutrons are smaller, so can travel farther through a medium without potentially interacting, down to betas and gammas, extremely small, will travel far through most things
For the second point, the similarities in mass come into play, consider you are at a bar playing pool, and you hit the cue ball directly into the 8 ball, the cue ball stops dead in its tracks, and the 8 ball is accelerated at the same speed away as the cue ball was traveling, this is maximum energy transfer. In a molecular world of a cue ball flying through 10 billion 8 balls packed closely together, its energy will be diminished and lost throughout quickly. This is the case for water and neutrons, water contains a ton of hydrogen, which at its base is a single proton, which has basically the same mass as a single neutron. Now imagine you went outside the bar and threw the cue ball at the brick wall of the bar, it would bounce off and basically retain all energy, this is the case for shielding like lead, very very heavy and large atom, but very dense, so instead of removing energy from radiation, it just directs it back. In general though, water has the right mass, and right density to be a good shield.
Yes you are made of water, but you are not made of tens of feet of water surrounding every individual cell, radiation will still get through to your important stuff. These small particles always have a chance to pass your skin, and blood and water and interact with a cell. Generally the simple way of thinking about radiation damage is the particles hit your DNA and knock pieces out of it, ripping pages out of the instruction manual of you per se, so when those cells reproduce, they get it wrong, and then those daughter cells are also messed up, and they reproduce wrong too now, after generations of cell reproduction then you have a mass of messed up cells. Cancer.
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