Explain why X-rays are used to produce images of bones inside the body, rather than of soft tissue.

AQA A-Level Physics (7408) — 3.10.5 X-ray Imaging · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

X-rays are a form of electromagnetic radiation used in medical imaging. When X-rays pass through the body, different tissues absorb different amounts of radiation.

Model answer (5 marks)

X‑rays are absorbed by dense materials such as bone.
Soft tissue is mostly water and therefore transmits X‑rays.
Where bone absorbs the X‑rays, the radiation does not reach the detector, producing a bright or white shadow on the image.
Where soft tissue transmits the X‑rays, they reach the detector and appear as a dark area.
The resulting contrast allows bones and any fractures to be clearly seen against the surrounding soft tissue.

Examiner tips

Use the exact terms ‘absorbed’, ‘transmitted’, ‘shadow/white area’, ‘dark area’ and ‘contrast’ as the scheme rewards.
Show the cause–effect chain: dense material → absorption → no radiation reaches detector → bright shadow.
Mention that soft tissue transmits → radiation reaches detector → dark area, giving contrast.

Common mistakes

Confusing ‘white’ with ‘dark’ for bone; students often write bone appears dark.
Failing to explain that soft tissue transmits X‑rays, not absorbs them.
Omitting the link between contrast and the ability to see fractures.

Mark scheme (5 marks)

X-rays are absorbed by (dense) materials such as bone
X-rays pass through / are transmitted by soft tissue
Where X-rays are absorbed (by bone), they do not reach the detector / photographic film, leaving a shadow / white area
Where X-rays pass through soft tissue, they reach the detector and produce a dark area on the image
This contrast means bone (and fractures/breaks in bone) can be clearly seen / distinguished from surrounding tissue

Key terms in this question

X-rays · soft tissue