If the focal spot to film distance is increased from 20 cm to 40 cm, how is the intensity of radiation affected?

• A. It is reduced by ½
• B. It is reduced by ¼
• C. It is reduced by 1/3
• D. It is reduced by 1/5

Answer: (B)

The intensity of radiation is influenced by the inverse square law, which states that the intensity of radiation is inversely proportional to the square of the distance from the source. When the distance from the focal spot to the film is increased, the intensity decreases according to this principle.

In this scenario, when the distance is increased from 20 cm to 40 cm, the new distance is twice the original distance. According to the inverse square law, if the distance is doubled, the intensity of the radiation is reduced to one-quarter of its original value. This can be expressed mathematically as:

I2 = I1 / (d2/d1)²

If you substitute the distances, you find:

I2 = I1 / (40 cm / 20 cm)²

I2 = I1 / 2²

I2 = I1 / 4

Thus, the intensity is reduced to one-quarter of the original intensity. This understanding directly aligns with the selected answer regarding the reduction in intensity due to the increased distance.