# How can gamma rays affect a human body if they can move through atoms without colliding with them?

I found this answer on this SE about why mirrors don't reflect gamma rays, and the answer says that it is because gamma rays are so, so small that they "see mostly empty space between the atoms of the solid.".

If that is true, so gamma rays can move through atoms without colliding with them, why can gamma rays affect people enough that they are said to be dangerous? If they are so small, that would mean it's very unlikely for a gamma ray to accidentally touch an atom, so it would be a very rare situation, which the body should be able to fix without problems due to it's unlikeliness, right?

• I'm voting to close this question as off-topic because this is more about biology than physics. – tpg2114 Jun 9 '18 at 22:19
• @tpg2114 I completely disagree. The fact that some words from other disciplines appear, doesn't mean anything. The actual question behind these lines is "why should a gamma photon hit an atom of a lattice". Physics are used everywhere, and that's because a physicist thinks about skin as "a lattice of atoms", and that's very physical, and useful. Creating physical models like this is the way of science to keep going on. – FGSUZ Jun 9 '18 at 22:24
• @FGSUZ I'd say tpg2114 is somewhat right in that this question is (or, was) touching on aspects of biology that are off topic here. We do expect questions to be narrowed down to just the physical core of what they're asking - the fact that physics is used everywhere doesn't make everything on topic. Sure, the underlying question may be "why should a gamma photon hit an atom of a lattice?" but in that case the question should be edited to ask that. In this case it happens to be easy enough to make that edit, and I've done so. – David Z Jun 9 '18 at 22:34
• too late, I answered it. – JEB Jun 9 '18 at 22:35
• Well, if the rules say that the question must be so explicit, then it's okay. However, we all are trained to identify such underlying questions; and, for me, anything is physical as soon as (and only if) it's faced from a physical point of view, and this is. – FGSUZ Jun 9 '18 at 22:45

Based on your question: Yes, it is extremely unlikely that a gamma ray will hit an atom; however, Avogadro's number ($N_A$) is large: there are many atoms in your body, so it is likely it will interact.
Also: a single gamma interaction is not a problem, but for any appreciable source, there are many gamma rays. For instance, where I work we have a 20,000 Ci cobalt-60 source--that's $7.4\times 10^{14}$ decays per second, and there are about two gammas per decay (about 1.2 and 1.1 MeV, respectively).