Final answer:
Depressing the exposure switch on an X-ray machine initiates the acceleration and collision of electrons with a metal plate, producing X-rays.
Step-by-step explanation:
When the exposure switch on an X-ray machine is depressed, the first event that occurs is the acceleration of electrons in an evacuated tube by a high positive voltage. These electrons then strike a metal plate, typically made of copper, resulting in the production of X-rays. This process involves a high-voltage discharge that provides the electrons with enough energy to ionize atoms and generate x-rays, which can then pass through different types of materials to create images based on varying densities.