Ultrasound is a procedure as common as X-ray and those who have undergone this procedure know well that application of gel is a part of it. The gel is mostly water which is a medium of contact between the ultrasound probe and the body. The primary purpose of the gel is to relay the acoustic energy from the transducer to the tissue without passing through air at any point of time. The reason it is so important is that both refraction and reflection are reduced while it crosses an air interface. Before you jump on to the concept of refraction and reflection, you must learn about acoustic impedance.