Final answer:
The actual size of the nucleus is 0.006 mm when the nucleus measures 3 mm across in a photograph with a 500x magnification.
Step-by-step explanation:
The question is asking us to calculate the actual size of a nucleus based on the magnified size in a photograph. If the nucleus appears to be 3 mm across in a photograph that is magnified 500 times (× 500), we need to divide the measured size by the magnification factor to find the actual size.
To calculate the actual size, we use the formula: Actual size = Measured size / Magnification. By substituting the values into the formula, we get:
Actual size = 3 mm / 500 = 0.006 mm
Therefore, the correct answer is A. 0.006 mm, which represents the true size of the nucleus when not magnified.