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.

Acoustic mismatch arise at the boundary between two different media where reflection and refraction occur.

See also Snells Law.

(Doppler look angle) The angle of incidence is the angle at which the ultrasound beam strikes an interface, or the angle at which the Doppler beam intersects the blood flow. Doppler signals are best obtained at angles of 60° or less.

See also Snells Law.

Reflection of the sound beam occurs when it hits a boundary between materials having different acoustic impedance. The reflection (echo) is the portion of a sound that is returned from the boundary. The reflection time (the time taken for the wave to return to the probe) can be used to determine the depth of the object.
The reflection within the body produces the ultrasound image, but should be minimized at an ultrasound couplant to skin boundary where the couplant acts as an acoustic window through which the image is seen. The amount of sound waves, which are reflected back at the interface between two tissues is depend on the angle of incidence and the difference between the acoustic impedance values of the two tissues.
If the difference is great, a large part of the sound waves will be reflected back. If too much sound is reflected back and not enough waves are remaining to be able to penetrate the tissue, the imaging will be poor.
If the difference is small, a small amount will be reflected back. Enough sound signal remains to continue with ultrasound imaging.
If the ultrasound beam meets a rough surface or small object, the beam is scattered in all directions and only a small amount will be received by the probe.

See also False Distance Artifact, Target Strength, and Snells Law.