A beam of X-rays is attenuated as it passes through matter: I = I₀e⁻μᵛ, where μ is the linear attenuation coefficient. Dense, high-atomic-number materials like bone have a large μ and absorb strongly, casting the shadow that forms the image. Choose a material and a thickness and watch the transmitted intensity fall.
MissionCompare bone and soft tissue at the same thickness: the big difference in transmitted intensity is what makes bone show up.Streak 0Best 0
Bones show up on an X-ray image because, compared with soft tissue, they:
material · attenuation coefficient μSoft tissue · 0.20 cm⁻¹
thickness x5.0 cm
transmitted I/I₀37%
half-value thickness = ln2/μ3.5 cm
Each half-value thickness halves the intensity. Bone has a much larger μ, so far less passes through, giving the contrast in the image.