$A$ bat emits ultrasonic sound of frequency $1000 \; kHz$ in air. If the sound meets a water surface,what is the wavelength of
$(a)$ the reflected sound,
$(b)$ the transmitted sound?
Speed of sound in air is $340 \; m/s$ and in water is $1486 \; m/s$.

The frequency of the sound wave remains constant when it travels from one medium to another.
Given frequency,$\nu = 1000 \; kHz = 10^6 \; Hz$.
$(a)$ For the reflected sound in air:
Speed of sound in air,$v_a = 340 \; m/s$.
The wavelength $\lambda_r$ is given by $\lambda_r = \frac{v_a}{\nu} = \frac{340}{10^6} = 3.4 \times 10^{-4} \; m$.
$(b)$ For the transmitted sound in water:
Speed of sound in water,$v_w = 1486 \; m/s$.
The wavelength $\lambda_t$ is given by $\lambda_t = \frac{v_w}{\nu} = \frac{1486}{10^6} = 1.486 \times 10^{-3} \; m \approx 1.49 \times 10^{-3} \; m$.