University Physics with Modern Physics
Physicians use high-frequency ($f$ = 1$-$5 MHz) sound waves, called ultrasound, to image internal organs. The speed of these ultrasound waves is 1480 m$/$s in muscle and 344 m$/$s in air. We define the index of refraction of a material for sound waves to be the ratio of the speed of sound in air to the speed of sound in the material. Snell's law then applies to the refraction of sound waves. (a) At what angle from the normal does an ultrasound beam enter the heart if it leaves the lungs at an angle of 9.73$^\circ$ from the normal to the heart wall? (Assume that the speed of sound in the lungs is 344 m$/$s.) (b) What is the critical angle for sound waves in air incident on muscle?