A medical imaging device produces ultrasound by oscillating with a period of 0.400 microseconds. What is the frequency of that oscillation?

1. Frequency is the reciprocal of period, so f = 1 / T. 2. Convert the period: 0.400 microseconds = 0.400 x 10^-6 s. 3. f = 1 / (0.400 x 10^-6 s) = 2.50 x 10^6 Hz. 4. That is 2.50 MHz, well above the highest frequency a human ear can hear, which is why it is called ultrasound. 5. Quoting 0.400 x 10^6 Hz simply repeats the period digits and misses the reciprocal step. _Source: OpenStax College Physics (CC BY 4.0), Ch 16 "Oscillatory Motion and Waves", section 16.2 Period and Frequency in Oscillations_