Which equation expresses focal depth in cm as (diameter^2 / 40) times wavelength, with diameter in mm and wavelength in mm?

Which equation expresses focal depth in cm as (diameter^2 / 40) times wavelength, with diameter in mm and wavelength in mm?

• A. focal depth (cm) = diameter (mm)^2 / 40 x wavelength (mm)
• B. focal depth (cm) = (diameter (mm)^2 / 40) x wavelength (mm)
• C. focal depth (cm) = diameter (mm)^2 x wavelength (mm) / 40
• D. focal depth (cm) = diameter (mm)^2 x 40 / wavelength (mm)

Answer: (B)

The idea is to write the relationship so the order of operations is crystal clear and matches the stated form: focal depth in centimeters equals (diameter squared divided by 40) times wavelength, with both diameter and wavelength given in millimeters. Using parentheses around diameter^2/40 shows that you first form that factor, then multiply by the wavelength. This explicit grouping prevents any misreading that the 40 might apply to the wavelength or to the whole product, and it directly mirrors the stated relationship. In other words, take the diameter, square it, divide by 40, then multiply by the wavelength to obtain the focal depth. Note, if you’re checking units, you’d typically convert to centimeters at the end, but the key point here is the calculation order reflected by the parentheses.

The idea is to write the relationship so the order of operations is crystal clear and matches the stated form: focal depth in centimeters equals (diameter squared divided by 40) times wavelength, with both diameter and wavelength given in millimeters. Using parentheses around diameter^2/40 shows that you first form that factor, then multiply by the wavelength. This explicit grouping prevents any misreading that the 40 might apply to the wavelength or to the whole product, and it directly mirrors the stated relationship. In other words, take the diameter, square it, divide by 40, then multiply by the wavelength to obtain the focal depth. Note, if you’re checking units, you’d typically convert to centimeters at the end, but the key point here is the calculation order reflected by the parentheses.