1. Medical ultrasound uses frequencies on the order of 20 MHz. If the
speed of sound in tissue is 1500 m / s, what is the smallest object
which can be detected? If the penetration distance is approximately 200
wavelengths, how deep can this instrument penetrate?

2. Compute the velocity, wavelength and angular momentum of an electron in the n = 1 state of Hydrogen. Is Bohr's quantization condition satisfied? Is it meaningful to speak of the velocity of a standing wave?