The X-Rays question...

[Captain Indigo]

Please bear in mind that my knowledge of photography comes only from AS level Physics and a little Chemistry, and is limited to b/w pictures only, but from what I remember:-

The reaction on the film is caused by photons degrading the silver nitrate coating of the film, turning it black. This becomes the white areas on the developed photo. Greyscale areas are created where only some molecules of silver nitrate are degraded in an area, and not others.

As far as I know, all frequencies of light, including IR and UV, and further along the electromagnetic spectrum, are capable of carrying photons, and are, therefore, capable of producing images in b/w film. X-rays are still produced in the same way, although these images are fixed onto the film itself, rather than a separate picture. This is why bones and other X-ray inhibitors appear to be white - nothing has caused the silver nitrate to degrage in these parts of the film.

I would suspect that somewhere in the EM spectrum, the energy carried by the photons would cease to be strong enough to produce any change in silver nitrate, so lower frequencies would not be visible.

Yeowch! My brain hurts from dragging that up! I'm not sure how relevent it would be to colour film and digital photos though...