Acoustic impedance can be described as the amount of resistance that the Ultrasound beam encounters when it travels through molecules of the human body tissues. Anything with high level of acoustic impedance, like bones, have higher resistance to the beam passing through as compared to the material that has lower impedance like air or water. The acoustic impedance depends on the density of the tissue, its composition and the pace at which the ultrasound waves travel through it. Thus, it is safe to say that the impedance is a product of acoustic wave velocity and material density. When the ultrasound beam strikes an acoustic boundary, which is a point where two different interfaces with different acoustic impedance values are in touch with each other, like soft tissue and water, most of the acoustic energy passes through the tissue while some part of it gets reflected and some of it is refracted. When it gets reflected from the boundary the angle of reflection is same as the angle of incidence. The refraction angle, on the other hand, is different from the angle of incidence. Refraction is a change in direction and is best demonstrated by the example of a pencil in a glass of water.