Final answer:
A radiograph is blackened by a varying degree once it has been developed, which is when the exposure to radiation or light becomes visible as different levels of darkness on the radiograph.
Step-by-step explanation:
A radiograph is blackened by a varying degree once it has been developed. The process of development is akin to how photographic film is treated after exposure to light or radiation. When film is exposed to radiation, it is subject to a chemical change in the emulsion that causes it to darken upon development. Notably, the exposure is not just to visible light but can also be to ionizing radiation, as in the case of medical or dental X-rays. Photographic film was historically crucial in the direct detection of radiation, with the fogging of a film by Henri Becquerel being a notable example. The development process reveals images on a radiograph as areas of varying darkness corresponding to the degree of radiation exposure or exposure to light that the film has received.