Why do bones appear more clearly than soft tissue in an X-ray image?

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Diagram of the electromagnetic spectrum showing different types of waves arranged by wavelength and frequency, from gamma rays and X-rays to ultraviolet, visible light, infrared, microwave, TV/FM radio, shortwave radio, AM radio, and longwave radio. — X-ray The relationship of X-rays to other electromagnetic radiation within the electromagnetic spectrum.

Bones appear more clearly than soft tissue in an X-ray image because of the differential absorption of X-ray photons by different body structures.12

When a beam of X-rays passes through the body, materials of varying density, composition, and homogeneity absorb the radiation to different degrees. Bones are denser than soft tissues (such as organs and muscles), which causes them to absorb a greater number of X-ray photons. Consequently, bones cast the sharpest shadows on the resulting image, while soft tissues produce shadows of varying, less intense density.12