All intensities have units of?

• A. Pa
• B. J
• C. W/m2
• D. W/cm2

Answer: (D)

Intensities describe how much energy passes through a surface per unit time per unit area. That means the unit is power divided by area. In SI, that's watts per square meter (W/m^2). In many practical contexts, especially when dealing with beam measurements on small areas, area is expressed in square centimeters, so intensity is written as watts per square centimeter (W/cm^2). This is just a different but equivalent way to express the same quantity—the numerical value scales by 10^4 because 1 m^2 equals 10^4 cm^2. So expressing intensity as W/cm^2 is common in fields like ultrasound or optics, and is the form this item uses. The other options refer to related but different quantities (Pascals for pressure, joules for energy) or to the SI form (W/m^2), which is correct in general but not the preferred unit shown here.