Snell’s law and Refraction of wave

  Snell's Law describes the relationship between the angles and the velocities
                      of the waves.
                      Snell's law equates the ratio of material velocities V₁ and V₂ to the ratio of
                      the  sine's of incident (Q₁) and refracted (Q₂) angles,
                      as shown in the following equation
                                    
          

แหล่งข้อมูล : 

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/refractionsnells.htm