CAMRT Radiography Practice Exam

According to the inverse square law, if the exposure is 400 mR at 90 cm, what will the exposure be at 180 cm?

1. 200 mR

2. 100 mR

3. 400 mR

4. 80 mR

The correct answer is: 100 mR

The inverse square law states that the intensity of radiation is inversely proportional to the square of the distance from the source. This principle can be mathematically expressed as: \( I_1 / I_2 = (D_2^2) / (D_1^2) \) where \( I_1 \) and \( I_2 \) are the radiation exposures at distances \( D_1 \) and \( D_2 \), respectively. In this problem, the initial exposure is 400 mR at a distance of 90 cm. When the distance is increased to 180 cm, we want to find the new exposure. First, we can set up the relationship based on the law: 1. \( I_1 = 400 \) mR 2. \( D_1 = 90 \) cm 3. \( D_2 = 180 \) cm Now we can calculate \( I_2 \): \[ I_2 = I_1 \times \left(\frac{D_1^2}{D_2^2}\right) \] \[ I_2 = 400 \times \left(\frac{90^2}{180^2