A protective polymer lining is bonded to the inside of a storage vessel. A pulse-echo device sends ultrasound into the lining and receives echoes from its front and back surfaces.
The ultrasound has a frequency of 4.0 MHz and a wavelength of 0.60 mm in the polymer. The echoes arrive 18 μs18\ \mu\text{s}18 μs apart.
Calculate the thickness of the polymer lining.