An engineer calibrates an ultrasonic transducer in a testing bath by measuring the wave profile along its path of propagation.
Using a movable probe, she records the distance, ddd, between the peak of the second wave crest and the minimum of the seventh wave trough, as shown in the diagram below.
What is the wavelength of this ultrasonic wave?
18.6 mm18.6\text{ mm}18.6 mm
22.0 mm22.0\text{ mm}22.0 mm
24.2 mm24.2\text{ mm}24.2 mm
26.9 mm26.9\text{ mm}26.9 mm