# Why don't X-rays penetrate the bone?

My brother asked the simplest question ever: why do X-rays not penetrate the bone?

I know it is something to do with the compact molecular structure of bone. What I really need to know is, what is the reason for this? What is the atomic distance between bone atoms that prevents X-rays going through them?

• I'm not sure whether it's true that the x-ray wavelengths used for medical x-rays can't penetrate bone. That's not necessarily the case. All that is required for imaging is that there is a contrast between the x-ray absorption of bone and that of human tissue. And I believe that the x-ray absorption factor of a material depends not so much on the distance between the individual atoms of the material but more on the electron density of the material. – user93237 Mar 19 '16 at 3:10

• @Ruslan: also very important is the higher binding energy of the electron which greatly increases the cross-section, which goes as approx $Z^2$. It's not just increased density because overall the photoelectric effect goes as approx $Z^3$. – tom10 Mar 19 '16 at 14:10