Australian Dental Council (ADC) Practice Test

The intensity of radiation follows the inverse square law, which states that the intensity of radiation is inversely proportional to the square of the distance from the source. This means that if the distance from the source (in this case, the focal spot) is doubled, the intensity becomes one-fourth of its original value.

When the focal spot to film distance is increased from 20 cm to 40 cm, the distance has effectively doubled. According to the inverse square law, the calculation for intensity is as follows:

If the original intensity is I at a distance of 20 cm, then at 40 cm, the intensity becomes:

\( I' = \frac{I}{(2)^2} = \frac{I}{4} \)

This calculation confirms that the intensity of radiation is reduced to one-fourth of its original value, which corresponds to the answer provided. Hence, knowing this principle helps in understanding the relationship between distance and intensity of radiation in radiographic practices.