As the sound travels through a relatively homogeneous medium it propagates in essentially a straight line. When the sound reaches an interface a part of the incident beam is reflected, and a part is refracted (transmitted). Snells law governs the direction of the transmitted beam when refraction occurs:
sin qt = (c2/c1) x sin qi (qt is the transmit and qi is the incident angle)
The amount of sound that is reflected depends on the degree of difference between the two media; the greater the acoustic mismatch, the greater the amount of sound reflected. In addition, the amount of ultrasound reflected or refracted depends on the angle at which the sound beam hits the interface between the different media. As the angle of incidence approaches 90°, a higher percentage of the ultrasound is reflected.

See also Sonographic Features.

The refraction is the change of the sound direction on passing from one medium to another. In ultrasound, refraction is due to sound velocity mismatches combined with oblique angles of incidence, most commonly with convex scanheads. When the ultrasound wave crosses at an oblique angle the interface of two materials, through which the waves propagate at different velocities, refraction occurs, caused by bending of the wave beam.

See also Refraction Artifact, Acoustic Shadowing, Acoustic Mismatch, and Duplication Artifact.