What is the covering of a radiographic film made of?

The correct choice is that the covering of a radiographic film is made of silver bromide crystals. Radiographic film works by using these crystals, which are light-sensitive and play a crucial role in capturing the image. When exposed to radiation, such as X-rays, the silver bromide crystals undergo a chemical change that creates a latent image on the film. This image can then be developed into a visible image, allowing for diagnosis and analysis.

In the context of radiographic film, the other materials mentioned serve different purposes. Lead foil is typically found in the packaging of the film to protect it from ambient light and scattered radiation, but it is not part of the film itself. Plastic fiber refers to materials that may be used in various components of the film's packaging or storage but do not constitute the film covering. Film adhesive, while important for binding layers in the film or for other purposes, is not the primary material for the film's covering either. Thus, silver bromide crystals are indeed the key component in the construction of a radiographic film and are central to its function in radiography.