Why is it that optical waves get scattered or absorbed when directed into a human body yet X-rays pass straight through the body? The only difference between them is X-rays have a smaller wavelength so how does that enable to pass through a medium unimpeded?
Is it something to do with optical waves actually being particles in the form of photons? (And why do optical waves get considered as photons yet X-rays don't?
Best Answer
Well, the starting point is that light-matter interaction is an immense field of physics. Even if we focus only on a particular kind of radiation the effects it can produce are a lot, depending on the elements with which interacts, and on how they are combined together (for example if they are free atoms, or bounded together forming molecules or solids). Now, we can say that the intensity of a beam of monochromatic waves that encounters an object of width L, drops exponentially as it crosses that object, $$ I(x)=I_oe^{-\eta x}$$ where x is the distance the beam has covered, and $\eta$ is called the absorption coefficient, which encloses the mechanism of interaction itself, so it is different for X-rays and visible light. Now, we can roughly say that when a beam of visible light interacts with the tissues of the human body, $\eta$ has a magnitude big enough so that light is entirely absorbed, and in part re-emitted, in a few nanometers (billionths of a meter). However, when we talk about X-rays, they are thousands of times more energetic than visible light, due to the energy-frequence relation, which is $$ E=h\nu =h\frac{c}{\lambda} $$ So the shorter the wavelenght, the higher the energy of the photon. Remember that all kinds of electromagnetic waves have a quanto-mechanical description based on the photons, which are the quanta of energy carried by the electromagnetic field, but anyway. Turning back to X-rays, their energy is high enough to travel across the body tissues without it being absorbed; this is because the energy of X-rays is of the order of the atomic levels of the heaviest elements, which are scarcely present in a healthy body. The majority of human tissues are formed by light elements, such as carbon, oxygen, hidrogen, etc. that are unlikely to absorb an X photon. So they are free to cross the body, everywhere but tissues as bones, composed of elements that can "easily" absorb an X photon.