Final answer:
To calculate the exposure in mAs at 40 SID when it is 5 mAs at 30 SID, we apply the inverse square law of radiation. The result is approximately 2.81 mAs for the required exposure at 40 SID.
Step-by-step explanation:
To calculate the mAs exposure needed when changing the source-to-image distance (SID), we can use the inverse square law of radiation, which states that the intensity of the radiation is inversely proportional to the square of the distance from the source. If the original exposure at 30 SID is 5 mAs, and we want to find the exposure at 40 SID, we would use the following formula:
Intensity1 / Intensity2 = (Distance2 / Distance1)^2
Where Intensity refers to the mAs value, and Distance refers to the SID. Substituting the given values into the formula:
5 mAs / x mAs = (40 SID / 30 SID)^2
After solving the equation for x, we find that:
x mAs = 5 mAs * (30/40)^2
x mAs = 5 mAs * (0.75)^2
x mAs = 5 mAs * 0.5625
x mAs = 2.8125 mAs
So, the exposure needed at 40 SID would be approximately 2.81 mAs.