Refraction will not be observed in a sonographic image under which condition?

Refraction will not be observed in a sonographic image under which condition?

• A. The sound beam is perpendicular to the interface.
• B. The sound beam strikes the interface at an angle greater than 45 degrees.
• C. The sound beam strikes the interface at an angle less than 45 degrees.
• D. The sound beam changes propagation speed at the interface.

Answer: (A)

Refraction depends on Snell’s law: the direction of the transmitted beam bends when it crosses an interface at a nonzero angle and the wave speeds differ in the two media. If the beam is perpendicular to the interface, the incidence angle is zero, so the transmitted angle must also be zero. The wave passes straight through, so there is no bending visible as refraction, even though the speed changes in the second medium. In ultrasound imaging this means you won’t see refraction when the beam hits the boundary head-on. Refraction occurs only when the incidence is oblique; the exact angle (greater or less than a particular value) isn’t a fixed threshold, and a speed change alone doesn’t cause bending unless the incidence is not normal.

Refraction depends on Snell’s law: the direction of the transmitted beam bends when it crosses an interface at a nonzero angle and the wave speeds differ in the two media. If the beam is perpendicular to the interface, the incidence angle is zero, so the transmitted angle must also be zero. The wave passes straight through, so there is no bending visible as refraction, even though the speed changes in the second medium. In ultrasound imaging this means you won’t see refraction when the beam hits the boundary head-on. Refraction occurs only when the incidence is oblique; the exact angle (greater or less than a particular value) isn’t a fixed threshold, and a speed change alone doesn’t cause bending unless the incidence is not normal.