Australian Dental Council (ADC) Test 2025 – 400 Free Practice Questions to Pass the Exam

The inverse square law states that when the distance between the source of radiation and the film is doubled, the exposure received by the film decreases to one-fourth of the original exposure. This principle can be applied to calculate changes in exposure time due to changes in distance.

In this scenario, the distance changes from 200mm to 400mm, which is a doubling of the distance. According to the inverse square law, when the distance is doubled, the intensity of radiation reaching the film is reduced to 1/4 of its original value. Consequently, to maintain the same exposure to the film, the exposure time must be increased by a factor of four.

If the original exposure time at 200mm was 0.25s, doubling the distance to 400mm means the exposure time would need to be multiplied by 4, resulting in an exposure time of 1.0s. Therefore, this reasoning justifies that the exposure time, when the distance is increased resulting in a change from 200mm to 400mm, would indeed be 1.0 seconds.