Radiograph

[Tigger17]

Why do bones appear white on a radiograph?

[jomifl]

The more x rays that a silver iodide based x ray film receives the darker it is when developed. As bone is denser than the softer tissues it absorbs more x rays so the film behind the bones is partially shielded from the x rays thus doesn't get so dark.

[Tigger17]

Thanks

[Lborobrewer]

Currently very few radigraphy departments use film, an electronic plate is used instead, but the argument is the same, the higher the level of transmitted radiation the darker the image. The original radiograph is analogous to a photographic negative.
The advantage of the electronic version is that it can be manipulated by software to alter the image and hence improve visibility of different structures.