The critical angle (θc) describes a refractive phenomenon in ultrasonography, whereby a specific incident angle of the sound beam will result in 'total reflection' 1. In this situation, the transmitted sound beam, will run parallel to the boundary of the second tissue without penetrating it 1. This phenomenon will only occur when the sound beam is travelling from a material with low propagation speed to a material with higher propagation speed (i.e. c2 > c1) 1.
The critical angle may be calculated by substituting the transmitted angle for 90 degrees in Snell's law, thereby simplifying the equation to sin θc = c1/c2 1.
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