DANB Radiation Health and Safety (RHS) Practice Exam

If the target film distance is doubled while keeping film speed and machine variables constant, how will the exposure time change?

1. increase by ninefold

2. decrease by ninefold

3. increase by fourfold

4. decrease by fourfold

The correct answer is: increase by fourfold

When the target film distance is doubled while keeping film speed and machine variables constant, the exposure time will increase by fourfold. This change is rooted in the inverse square law of radiation, which states that the intensity of radiation decreases as the distance from the source increases. Specifically, when the distance from the source of radiation (in this case, the X-ray tube) is doubled, the intensity of the X-rays reaching the film is reduced to one-fourth of the original intensity. As a result, to maintain the same level of exposure on the film, the exposure time must be increased proportionately. Since the intensity is reduced to one-fourth, the exposure time must be increased by a factor of four to achieve the same level of film exposure. The other options suggest either an incorrect increase or decrease in exposure time, which does not align with the fundamental principles of radiation physics. Thus, the correct understanding of the relationship between exposure time and distance under constant conditions leads to the conclusion of a fourfold increase in exposure time when the target distance is doubled.