Here, $\upsilon = 1000\ \mathrm{kHz}$ $= 1000 \times 103\ \mathrm{Hz} = 106\ \mathrm{Hz}$; speed of sound in air, $v_a = 340\ \mathrm{m s}^{-1}$; speed of sound in water, $v_w = 1{,}486\ \mathrm{m s}^{-1}$ (a) The reflected sound : After reflection, the ultrasonic sound continues to travelin air. If $\lambda_a$ is wavelength in air, then $\lambda_a = \frac{v_a}{\upsilon} = \frac{340}{10^{6}}$ $= 3.4 \times 10^{-4}\ \mathrm{m}$ (b) The transmitted sound : The transmitted ultrasonic sound travels in water. If $\lambda_w$ is wavelength of ultrasonic sound in water, then$\lambda_w = \frac{v_w}{\upsilon} = \frac{1{,}486}{10^{6}}$ $= 1.486 \times 10^{-3}\ \mathrm{m}$