Question

What's the frequency of 0.19-mm-wavelength ultrasound used in fetal imaging? the waves travel at 1.5 km/s?

Answer 1

The basic relationship between frequency, wavelength and speed of a wave is given by:
$f= \frac{v}{\lambda}$
where
f is the frequency
v is the wave speed
$\lambda$ is the wavelength

The ultrasound wave in our problem has wavelength of
$\lambda = 0.19 mm = 0.19 \cdot 10^{-3}m$
and speed of
$v=1.5 km/s = 1.5 \cdot 10^3 m/s$
So its frequency is
$f= \frac{v}{\lambda}= \frac{1.5 \cdot 10^3 m/s}{0.19 \cdot 10^{-3} m}=7.89 \cdot 10^6 Hz$