If an exposure time of 60 seconds is needed at 4 feet, what exposure time would be required at 5 feet for equivalent density?

To determine the exposure time required at a new distance while maintaining equivalent density, the Inverse Square Law is applied. This law states that the intensity of radiation (and thus exposure) is inversely proportional to the square of the distance from the source.

Given that the exposure time is 60 seconds at 4 feet, we can calculate the required exposure time at 5 feet as follows:

1. Determine the change in distance: The initial distance is 4 feet, and the new distance is 5 feet. This is a change from 4 to 5 feet.

2. Calculate the ratio of distances squared: The ratio of the distances squared (5 feet and 4 feet) is essential here. This can be computed as follows:

[

\text{Ratio} = \left(\frac{5}{4}\right)^2 = \frac{25}{16}

]

3. Adjust the exposure time based on the ratio: To find the new exposure time that results in the same density at 5 feet, you multiply the original exposure time by this ratio:

[

\text{New Exposure Time} = \text{Original Exposure Time} \times \text{Ratio} =