Which of the following has the longest Fresnel Zone?

The concept of the Fresnel Zone in ultrasonic testing refers to the area around the beam axis that contributes to the diffraction pattern received at the transducer. Its size is influenced by both the frequency of the ultrasonic wave and the diameter of the transducer. Generally, lower frequencies and larger diameters yield larger Fresnel Zones.

In this case, the option with the largest diameter (1-1/2 inches) and a relatively lower frequency (2.0 MHz) will produce the longest Fresnel Zone compared to the other choices. A larger diameter means that more of the wavefront can be captured, which contributes to a larger effective zone. Although it is at a frequency of 2.0 MHz, which is higher than some of the other options, the significant impact of the larger diameter on the Fresnel Zone size is the key point in determining its length.

Thus, the combination of a larger diameter and the reasonable frequency results in the longest Fresnel Zone in this scenario.