State Snell's law for ultrasound refraction and define the variables.

• A. sin θ1 × c1 = sin θ2 × c2; c1 and c2 are speeds of sound in media 1 and 2
• B. sin θ1 / c1 = sin θ2 / c2; c1 and c2 are speeds of sound in media 1 and 2
• C. θ1 / c1 = θ2 / c2; c1 and c2 are wavelengths in media 1 and 2
• D. c1 sin θ1 = c2 sin θ2; c1 and c2 are densities

Answer: (B)

When an ultrasound wave crosses a boundary at an angle, the frequency stays the same and the tangential component of the wavevector must be continuous. This leads to Snell’s law for sound: sin θ1 divided by c1 equals sin θ2 divided by c2. In other words, sin θ1 / c1 = sin θ2 / c2, where c1 and c2 are the speeds of sound in the two media.

Here’s what the variables mean: θ1 is the angle of incidence measured from the normal to the boundary in medium 1, θ2 is the angle of refraction in medium 2, c1 is the speed of sound in medium 1, and c2 is the speed of sound in medium 2.

That form is correct because the ratio of the sine of the angle to the wave speed remains constant across the boundary, reflecting how wavefronts bend when speed changes. The other options mix up these quantities or use incorrect dimensions (for example, multiplying angles by speed, or using densities instead of speeds), so they don’t describe the refraction behavior correctly.