https://www.gosh.nhs.uk/conditions-and-treatments/health-dictionary/health-dictionary-x/
Health dictionary - X
X-ray
X-rays allow the doctors to look inside your body, from the outside. The x-ray machine takes a picture, which helps them to see what's going on inside your body. The x-ray beams pass through soft tissue so looks black in the picture, but gets stopped by anything solid, such as bone, which shows white.