Electromagnetic and medium transparency

Question

What causes light to penetrate a medium?

For example, radio waves travel further in the atmosphere at longer wavelengths.

But, comparing visual to x-ray, x-rays which are shorter wavelengths penetrate human tissue further.

Answer 1

In every medium, the photons of electromagnetic radiation interact with the particles of the medium. And this occurs with varying intensity. X-rays, for example, interact only slightly with soft tissue, but in the bone area the photons are absorbed and re-emitted as longer wavelength photons.

Radio waves, where the number of emitted photons periodically increases and decreases, are particularly interesting. Just as the periodicity of these radio waves is capable of moving electrons back and forth in a receiving rod, EM waves can trigger phonons on surfaces or reflect them in the earth's ionosphere.